Since the y-pipe is made up of 2 2.5" pipes going into a 3" collector, I'm beginning to wonder if I could hack off the 3" and put a 2x2.5 into 3.5 collector, and run 3.5 the rest of the car. Has anybody done this or know where I could find a 2x2.5->3. I looked around online but didn't find much

This actually makes sense as the area of 2 2.5 pipes is 4.9*2 = 9.8. The area of a 3.5 pipe is 9.6.

If I can manage this, I could go with a high flow 3.5 cat, and a mufflex 3.5 back...

 

Flowmaster makes a 2 1/2 to 3.5" Y pipe. Part number is Y250350. If you order through Summit Racing the part numers is FLO-Y250350.

I have one on my car with dual cats and the Mufflex 3.5" cat-back system. For total performance order the Mufflex system with the Magnaflow 12909 muffler.

 

Sweet, that's exactly what I need. thank you sir. Now if I can just find a 45 degree 3.5" bend.... might have to hit the muffler shop for that.

 

looks like Summit's got that too: WLK-41429. Time to get the welder and the cutter/grinder out.