Snakeman's ability reverses stat changes, so it's not just Leaf Storm that'll do it; anything that drops your stats (Leer, Scary Face, Icy Wind, etc.) will raise them instead.
I think the Dream World abilities are an attempt not at making past Pokemon bulkier, but at making them a bit more competitive. Blaziken getting Speed Boost (and the superpowered Hi Jump Kick!) is definitely an attempt at giving him an edge over Infernape. Venusaur gets Chlorophyll. The big ones are Drought Ninetales and Drizzle Politoed. Of course, from a competitive standpoint, there are some useless ones, such as Frisk Wigglytuff, but for the most part, I think that many previous Pokemon will benefit from the Dream World.