-3 is 40% of stats, not 50%. Also, way to break Regigigas. If Slow Start works with stat stages like that all you need to do is run it next to Topsy-Turvy Malamar. Hit it with the Topsy-Turvy on turn one, and then Regigigas is suddenly at +3 and goes to +6 in three turns. And from personal experience I know that at +4 Regigigas is already powerful enough to OHKO almost everything in the entire game, except for defensive behemoths like Mega Aggron, Avalugg, Regirock, Mega Steelix, Multiscale Lugia, and Giratina-Origin. Regigigas at +4 has more attack than Snorlax has at +6. I've run Regigigas next to Simple Beam Xatu (as in, Xatu uses Simple Beam on Regigigas and Regigigas uses Swagger on Xatu in the same turn. Swagger gets bounced back and Regigigas goes to +4 due to its Simple ability). This would be way more powerful as it doesn't rely on the accuracy of Swagger, Malamar is bulkier than Xatu, Regigigas can attack in the first turn, it's not as Intimidate weak (Simple amplifies Intimidate impact), Regigigas can run something other than a Persim Berry, and Regigigas also gets a huge speed boost.
Alternatively, even without the Malamar, Regigigas gets Psych Up so it could simply use Psych Up on the first turn to negate its stat drops and then essentially become a superpowered Speed Boost pokémon that also gains attack boosts.