Peter - Yes, I think it is worth the effort over the OEM CAT. Since they all seem to be the same overall size (just different inlet/outlet sizes), I'd go with a 4" mostly because 3.5" is an oddball size to find adapters for. However, if you run 3" all the way back, you can probably expand the 3" pipe out to match the 3.5" diameter on the cat and weld it all together.
My only concern is that if it's actually a functional CAT and you have too low of temps in the CAT itself, it might start to clog, but given what you're doing with it, if it starts to plug up a bit on the way to the track, it'll be cleaned out after a couple laps
and you probably won't be too smokey anyway.
I'm the first to admit that 4" is probably no improvement over 3" (and heavier) but I would bet that 3" shows some measurable improvements over 2.5" particularly at high RPM's and over the length of the car that the exhaust runs.
You should still be able to get 3" over the rear axle even on a lowered car. The point where the exhaust goes over the axle is close to the pivot point on the frame - at this point the axle moves something like 1/5 the distance of the wheels so a 2" drop reduces your clearance by ~0.4" for the exhaust.
If you want quiet with no resonances, put the muffler in the OEM location Mine is a 4" ID with an 8" OD (the oem heat shield for the OEM muffler gets recycled - weight reduction
). The 4" ID tube is perforated with luvers pointing back - kind of like a cheese grater and the gasses flow across it the "easy" way. Given the fact that I can stick my entire arm through it, the flow restriction is negligable, however it does wonders for noise/resonances/drones. I assume that 3" would be the same
If you're really concerned with weight (and you're going to think I was dropped on my head when I was younger......), after the flex pipe/bellows, you could probably get away with running aluminum pipe - just use the stainless flat band clamps to make connections to the stainless parts and hold everything together. Put the CAT back at the OEM muffler location and figure out which tip/turndown combination works for you for drones/resonances.