High Flow Y-pipe-
Probably best power bang for buck and pretty painless (DIY or easy muffler shop install). Worth 6-10HP depending on type and if you get the kind with high flow cat or no cat (or keep stock cat like MSDS). About $230 for SHO-Shop Y-pipe with highflow cat which will be a bit louder than stock but not much. MSDS, must get from someone who bought the header/Y-pipe set but dosn't need the Y-pipe, about $200, uses stock act so only minimally louder than stock. No cat = louder.
Some people remove the resonator but this is noisy. Catbacks are good but more expensive mods and change the sound quality of the car, so maybe better to avoid if you want stock sound.
Stay tuned, we are just learning about bigger used Mustang 4.6L throttle bodies and MAFs which we can get cheap and may offer benifit but need dyno proof. Would hold off for now, Same with gutting precats and chips - maybe later when we have a bit more info or you have done more research.
Handling, think bigger rear sway bar - see Aussie SVTs group buy on 24mm rear bar. About $120 and addresses the 2 main issues with the SVT handling - understeer & body roll.
