haste..cap? That is your problem, haste is still the best stat, at worse it drops to about even with mastery for a short time after 30% but it is foolish to reforge to mastery and if you are talking about the global cooldown cap, no you will still do more damage. I can burst 40k+ dps with beserking+bloodlust at the start of fights in 25 man normals. wait for procs + Volcanic pot+beserking+refresh dots+archangel, etc is amazing. Well timed beserkings add several k dps for me even averaged out over a boss fight. Specially for bosses with burn phases like alysrazor or...every other boss to some extent. I'm sure with beserking +haste buffs +bloodlust my dots alone are pulling a good 100k+ dps when she is down. At least During beserking I Pull about 230k+ dps vs about 200k after it wears off and before my other buffs do. Remember haste buffs multiply with each other, so the more different haste buffs you have, the more benefit each one gives.
Beserking is king if used right by far. Goblin lets you hit haste marks easier, but keep stacking haste. Mastery isn't bad but don't prioritize it. Haste is a much more robust stat from fight to fight.
So yeah, go goblin if you don't think you can handle the added complication of timing beserking several times a fight, you will do fine. If you want maximum dps and flexibility of on call burst go troll.