I am going to be puting in a new system in the evo cause the speakers suck I had a bad *** one in my last car but the 2 12's in the back weighted a grip i am thinking of only getting a 10 or 12 to save some weight will 2 heavy *** 12's really hurt performance? if so by how much?

 

Depends on the size of the box too. The boxes weigh a lot. Two 12's will be around 50lbs. A box to hold them will be around 50-60lbs. The amp will be about 15lbs. Yes, it will hurt performance, in both straight line and track. Each 100lbs added is roughly a tenth of a second off the quarter mile.