how much of a power difference would i feel between a 2.5 in downpipe and a 3 in downpipe on d16z6 w/ a T3/T4 turbo?

 

The turbo is going to spool quicker. The bigger the difference in pressure and temperature, the faster the spool time.

 

but do you think it is cost-effective to have a 3 inch downpipe with a turbo D?

 

I havent built my DP yet, but I have a 2.5" exhaust and a 2.5" flex pipe.

Would it be benificial at ALL to use a 3" downpipe to the 2.5" flex?

 

Depends on the cost of it. I'm building a turbo D as well and thats one of the things I'm looking at. I think it creates emissions problems in the catalytic converter doesn't heat up quick, so it doesn't function anymore. I'm not sure if that happens when the cat and downpipe are too big or just the cat.