I think, if you're running an Impish set, enough Speed to outrun +ive Base 130s at +1 would be ideal, to avoid Aerodactyl's Taunt/Rock Slide and Jolteon's Tbolt.
On PS I run Impish @ 248 HP/244 Def/16 Spe
BP
Spikes
EQ
Protect/Swords Dance
In game, so, with the lvl cap at 50, the EVs have to...