Most tolerances today are good enough from the factory that you can't do a huge amount with it. Check out how little Car and Driver were able to eak out of their Nissan 350Z's 3.5 litre.
http://www.caranddriver.com/reviews/...ng+page-2.html
Balancing helps revs, reduces stress on the crank a bit, and helps efficiency a bit. For instance, my car's pistons and connecting rods all have identical bob weights (to the nearest gram I think.)
Also, this gets a more significant result from larger engines . . . the gain from a 6.6 litre is quite a bit larger (but given all other things equal, not proportionally larger due diminishing returns due to scaling) than what one would get from 3.8. I don't know how much the turbo could take advantage of the better tolerances and balance though. Building revs more quickly would be beneficial but that wouldn't make for a much larger peak.