Speed allows one to have the reaction time to dodge, block, and counter attacks. It may not make one stronger, but it can still close gaps better. That (and mobility) are a good part of what allowed Pikachu to beat Silvally in the first place. About Pika's inconsistencies, we've seen that many times outside of BW/SM (like losing to Gary's Electrivire like 2 days after beating Regice, being meh in the Johto league after an amazing performance against Clair, being meh against Morrison but great against Tyson, being able to tie with Latios after playing second fiddle at best throughout almost all of DP, or getting clobbered by Barbaracle and outperformed by a newly evolved Frogadier not long after beating Mega Lucario). Pikachu's strength vs Tobias is much higher than vs Paul going by what has been portrayed there.
About Silvally, yeah, I'll admit he doesn't have many feats, but a good part of his trouble against Melmetal was that Gladion knew nothing about him (and he fared much better once he knew more about how to deal with him), and he also has a few other good ones, like beating Lycanroc without much trouble and still having restraints (though this was Lycanroc's first post-evolution battle, so that's not much), 2HKOing Totem Kommo-o, and tanking Turtonator's IO (that was portrayed as really powerful to the point that everyone in the audience were covering themselves) just by switching discs. Though yeah, I carried myself a bit too far by saying Ground Silvally>Electrivire~Normal Silvally, it'd be more like Electrivire~Ground Silvally>Normal Silvally.
