I have read a bunch of strings on swapping in the 3.71 into cars that came with the 3.11 originally and had showed some performance gains (1/4 mile. 0-60) with the new gearing. I have a sts that already has the 3.71 and stock 16 inch wheels with 225/60's on them. What effect would it have with the final drive by putting on 225/40's? I know that my speedo would be off a bit and my ride height would be lowered a bit which would not be that bad side effect but I want to know if it would improve my 0-60 times or 1/4 mile time at all?

I think it would hardly be worth the expense. You might gain half a tenth in the 0-60, and maybe a full tenth in the 1/4 mile. (That's nothing like the 3-5 tenths you'd probably get from going from 3.11's to 3.71's.) Handling would be improved a little, but the ride would be noticeably worsened, and speed bumps would take on new meaning.

Yea I was looking to get 2 1/10ths out of it because thats all I need to break into the 13's!
I need two new front tires anyway so I am shopping for tires now.