The Macho Brace will only lower speed while battling. It does not affect the EV gain of the speed.
Heres a full set just cause I don't want to say EVs without moves and stuff.
Samurott (M) @ Life Orb
EVs: 64 Atk / 192 SpA / 252 Spd
Naive Nature (+Spd, -SpD)
- Hydro Pump
- Hidden Power Grass
- Ice Beam
My favorite set for Samurott. Hydro Pump is a good, strong STAB. Hidden Power Grass is coverage. Ice beam is more coverage specifically against Grass Types. With the Attack EVs, Superpower packs a punch and the Attack drop doesn't matter too much because it is a specially based set.
Hope I helped :)