v7 downpipe is virtually the same design and diameter as USDM, just cut into a different number of pieces and 1 fewer cat. IIRC the midpipe may be 0.25" bigger in diameter. but it will make hardly any difference and will not get you to stage 2.
get an aftermarket downpipe, it makes a huge difference and won't increase sound as drastically as an aftermarket catback. the only aftermarket catback I've heard that comes anywhere near stock sounding is Fujitsubo and they are pricey. Prodrive is NOT. Maddad Whisper maybe but I haven't heard one.