from my experience owning a Civic VTI-R, which i guess can be compared to ur high revvin sportivos.
when getting an aftermarket exhaust for the car, alot of research should be done. most local exhaust shops dont know much about our high revving, small displacement machines.
as far as i know, changing the muffler will yield little if not, no gains in power at all.
it's all noise!!!
changing the exhaust from cat. converter back, if done correctly will give some small gains, im talking like coupla kw's at the wheels. by correctly, i mean the right size piping etc. i'm not too familar with corollas and their factory piping sizes, however, you guys should find out what size piping the TRD system is, and tell ur local exhaust shop for some mandrel bent piping in that particular diameter piping.
the most gains are to be had in the header, matched with a hiflowing cat and well thought out catback piping + muffler, expect to get some decent gains!
when getting a catback exhaust, expect to lose a bit of torque down low, its not really noticeable, since civic vti'rs are gutless down low anyways (im assuming ur corollas are the same), however topend, is where u'll find gains, besides, when u want ur car to go quick, ur just playing with top end anyways right, ie in the LIFT ZONE! :)
so yeh - that is my opinion based on my own experiences when researching to upgrade my factory system. its easier for us honda boys as there are plenty of JDM bolt ons made for our cars.. however, toyota's aftermarket is somewhat smaller in comparison, so do ur research !!!
PS: sorry for the overly long post.