I have a 02 WS6 with an off road Y-pipe, by taking the cats off would I gain more up top then I loose down low? also would the car have to be tuned to get the most out of the off road Y-pipe?

 

its a gain... not a significant one at all... don't be thinking your car is so much faster cause you don't have cats, there is no loss involved.
and no... abosolutly no tuning is necessary

edit: on the tuning note... you will however need o2 simulators or a "tune" that will delete the code your car is gonna give when you take out the cats. thats just to get rid of the pesky ses light tho... it will still run the same

 

You won't see a big gain, the y-pipe would be more beneficial if accompanied by a pair of headers.

 

I agree with 2000GTP's comment. I also would like to add that IMO if anything by taking out the cats, you would gain power down low, not so much in the top end. Any power gained will be minumal though.

Back when I had just bolt ons to my Z, I noticed a gain in MPH in my 1/4 mile times when I put the MORE restrictive plate in my Borla exhaust, meaning I had an increase in HP from adding restriction to my exhaust. A stock LT1 needs some back pressure. If you make it to unrestrictive you can loose a few HP. At least thats what I found to be true with my bolt on car at the time.