Yeah you'll need a power adder if you want 400 to the wheels. 350 at the crank is about as much as you'll see N/A unless you start getting exotic (and expensive).
Seems like you've got the right order of priorities sorted out. The PBR calipers are inexpensive and allow you to keep the factory 10.8" rotors; the next upgrade up from there are the 13" rotors and calipers which require a hub swap or redrilling of the rotors to fit (they never made 13" rotors in our bolt pattern).
Consider an aluminum one-piece drive shaft to go with the higher gears. The increased drive shaft RPM at highway speeds will create annoying vibration that has been known to break things.
Also consider higher stall. The 11" converter from the SN95 GT (or Mark VIII and Marauder) is a good, inexpensive upgrade. Otherwise get a 10" 3200-3800 stall (depending on your engine build cam choice). The higher stall lets you skip the torque-deficient lower RPM range of the motor and get right into the powerband when accelerating. With a heavy car like the MN12, this + gears give the biggest bang for the buck, hands-down.
Look through Jerry's Thesis and the J-mod articles for a place to start with the trans. The Jmod will help the factory trans cope better with more aggressive driving and a little more power, and reading the article will also bring you up to speed on the weak points of the trans. The roller clutch has a habit of grenading when doing driving that promotes aggressive shifts from 4th or 3rd into 2nd gear.
FWIW, my "build" progress went roughly like this:
Dyno measuring point 1 (ca. 2011):
PI intake + PI cams, 02 GT MAF sensor, intake and air tube with 2.5" duals and X-pipe. Tuned for 93 octane: 212 RWHP
Net gain of ~45 RWHP over stock
Dyno measuring point 2 (ca. 2013):
Same as above, added on:
MHS ported stage 2 heads, Bullet stage 2P cams, ported factory manifolds, ported 70mm TB and plenum: 255 RWHP
Net gain of ~45 RWHP over previous combo and ~90 RWHP over stock
Dyno measuring point 3 (2020):
Same as above, changed:
MAC customized long tube headers, tune tweaks: 283 RWHP
Net gain of ~30 RWHP over previous combo and ~120 RWHP over stock
I could probably break 300 RWHP with a cam and intake change. The car is plenty fun as-is though.