Keith,
I put a high flow cat on my IS 200, can feel a big difference mid - top end. My launch from stop is still a bit weak though, not sure how to address that one. I am still in 2 minds whether to do the manifold to the cat 2.5" to finish the whole system. Would be very pricey i would think and am not too sure how much benefit i would get on 1G-FE.
I dont think the V8 would have a higher flow because it is a V8. Why would it need to? It is a v8 after all. Could be wrong though.
Cheers,
MattD