2.5" MANDREL BENT won't be much of any restriction to a car up to 500-550 HP. it'll be much quieter, too. another option is to go 3" to the mufflers and then 2.5" muffs and tails, as by then the exhaust gasses has contracted and cooled enough to not need all the volume.

a good article:

http://www.pontiacstreetperformance.com/psp/exhaust.html

pg. 2, they only lost like .05 ET going from 3" tails to 2.25" tails....