As long as its made clear that Perversity > not running Jaroda > Overgrow. Just point that if its available Perversity Jaroda is the best option, and if its not Jaroda is outclassed by pretty much everything.
Hes just a worse Sceptile without Perversity.
I think that depends on how you use it. Sceptile has better offence, but Serperior has much better hit-taking capability, whereas Sceptile can take, well, magikarps tackle, and, well, Luvdiscs surf, and, that's about it. Okay, I'm exaggarating Sceptiles fragility here, but you get the picture. Without Perversity Serperior isn't useful as an offensive beast. It simply hasn't got the base stats for it, but it can do other things.
I like this grass type. Finally a fast grass type that can actually take hits (glares at Meganium, Torterra and Venusaur for being slow and Sceptile for being fragile). I've already seen a lot of seeders here, and a few calm minders, even defensive ones, but no combination of the two (except one on page 5 without Giga Drain) for uber walling capabilities:
ZOMG It's a Calm VAMPIRE Snake, it drinks my HP, it just won't die:
Serperior @ Leftovers / Big Root
Trait: Overgrow
Bold nature (+Defence, -Attack)
EVs: 252 HP, 252 Def, 4 SpDef
-Calm Mind
-Substitute
-Leech Seed
-Giga Drain
Giga drain gained a neat boost in the generation shift going to 75 base power, and with 10 PP at the base it also has the same PP as Energy ball, so it's similar in power and PP. In exchange for 5 base power you heal 50% of the damage (or 65% with Big Root). It won't destroy foes in single hits (except perhaps Swampert), but it is a good move and an important building block in your defensive castle.
Subseed also heals and stops foes from bringing you down.
Calm Mind is neat, haven't seen much grass types that get it (apart from part psychic types like Celebi, which suffer from loads of weaknesses). It boosts your special defence so you are tougher to bring down, and it boosts your special attack so you get stronger Giga Drains that heal more, so you get tougher to break down.
For cartridge players, you don't absolutely need the TM for either Substitute or Calm Mind, you can breed Calm Mind onto the snake from Stantler, Infernape, Lucario, Mienshao or of course Smeargle, whilst for Substitute you can breed from Kecleon, although for the combination you need Smeargle.
Leftovers heals even more and is a pretty obvious option, but
Big Root is a great alternative, and likely even better, increasing the healing power of Leech Seed and Giga drain by 30%, certainly worth considering.
Overgrow is also neat on this set. Usually offense boosting natures like that aren't useful on walls that just heal heal heal, but if your foe (somehow) manages to get your hitpoints low, your Giga Drain gains 50% power and heals much more so you get back up and running.
Ideally you'd want to run this set with Paralysis support. Serperior gets Glare, which is better than the Stun Spore most grass types get, but there is simply no space on the moveset for it (or you turn it into a defensive parashuffler by dropping calm mind and relying on subseed to cause switches, which also has potential with slow hard-hitters like Heatran or Metagross) Without speed investments it'll be outran by a lot of pokémon from lower speed tiers (like Salamence or Gyarados) that do have speed investments, so paralysing much with some parashuffler to slow things down helps out since you want to be fast when subseeding to ease prediction and also to take the most out of Overgrow Giga Drain.
EV's and Nature
Max Def, Max hitpoints to maximize physical bulk whilst Calm Mind helps out on special bulk, although it's viable to drop some bulk (preferably from HP since leech seed and giga drain don't calculate on your own hitpoints so you don't lose out on the healing/damage difference) to gain speed for better subseeding.
So 252 defence, 4 HP, 252 Speed is also a good EV spread, outrunning Base 112s on neutral natures, although very painfully losing to base 100s with speed boosting natures.
With Timid and the speedy EV spread, it also outruns base 128s with neutral natures and base 112s (and thus base 100s) with speed boosting natures, although just losing to base 130s with neutral natures. You could drop the speed EV's a bit on Timid slightly but not much. Something like:
Timid, 252 defence, 228 speed, 28 hitpoints, outruns base 110s with speed boosting natures and outruns everything up to base 125 with neutral speed natures.
flaws
The weakness of this set lies in that it only has grass offence. Opposing grass types with neutral or super effective moves are immune to leech seed, take much less damage from Giga Drain, and can severely destroy it's staying power. Venusaur or Tropius for example have Super effective STAB moves, enough bulk to easily take boosted Giga Drains with their resistances, even on overgrow, and are immune to leech seed. Ferrothorn takes nothing from Leech Seed or Giga Drain, and can set up easily, although it can't really directly threaten Serperior since it gets stalled out of Gyro Ball PP too easily because of substitute, and could encourage the snake to set up more Calm Minds. Liquid Ooze also destroys this set if you switch it in on Giga Drain, especially when it hurts the most when Overgrow is active, since then Serperiors hitpoints are lowest whilst liquid ooze will damage the most. It also needs time to become strong. After several calm minds it becomes incredibly tough to take down, but if you don't give it that time it is easier to take down. Toxic also deserves a mention. No matter how much it heals each turn, eventually Toxic deals so much damage it's entire HP bar goes down in a single turn. It doesn't get around toxic, or it has to switch to an aromatherapist or heal beller and forcefully lose it's calm minds. Hard hitters that can break substitutes can also destroy it although they often can't switch in. Cloyster for example heavily fears it's grass offence, especially special side, but Serperior has every reason to fear Skill-link Icicle spear.
Partners. Parashufflers are good, as mentioned before. Something to destroy Grass types is also good. Salamence has fire moves and destroys grasses. Heatran destroys grass types and also resists all of Serperiors weaknesses. Aqua ring passers are also fun for even more healing force. I'm not sure if Big Root also boosts Aqua ring if it gets passed from another pokémon though, would be extra useful if it does.