That's a Buddy Club Spec 2 catback exhaust. I have one for my hatchback. Yes, it's super loud, and terminates just short of the USDM bumper, but it's awesome and super light. I may have to weld a muffler/additional resonator in-line, but I do like the simplicity of it. edit: Mine is also under-axle....

Make sure you bleed the system as best as you possibly can, seems like that may be the issue. I've had these issues as well, but turns out my radiator was leaking slightly. After replacing the radiator and both coolant hoses and bleeding the system for 20 minutes, the car runs like a dream!

What ECU are you running? Are you running stock bluetop injectors/electronics/computer?

I'm currently running a TRD 1.5way LSD and it is awesome for autocross/drift/etc. I have had zero issues controlling the car in any situation I've put it in. Highly recommend!