I think ultimately headers are what you have to worry about for back pressure. from the cat on is what I know should be free flowing.
get the EMS brand headers and pipe, then what ever muffler you like and you will be up in HP and neat on sound.
1990-1995 Toyota MR2 5sfe headers - $159.99 : EMSPowered, High Quality, Great Service, Low Prices
-H