I tried both the 11:1 & 12:1 pistons & the 12:1 definitely improved the bottom end. The proof is in the pudding, see my gallery pics for dyno graphs.
As you can see, I got more than a few hp & lb-ft torque gain on the bottom end when I upgraded from an 11:1 stock bore to a 12:1 stock bore piston.
Of course, the all around head porting influenced the bottom end gain, but it wouldn't have been so dramatic without the 12:1 piston. They really work hand in hand, the same way the stage I cam takes advantage of both. I ran pump gas (92-93 octane) with the 11:1, but I mix 1/3-1/2 race VP red (105 octane) for an equiv. of about 97-99 octane with the 12:1. Despite the fact that some people run pump gas with the 12:1, it really depends on altitude & jetting & knowing what you are doing. I wouldn't risk it, at least not until she's broken in!