I have been running the MP3 ECU for about six months. Bought it from SpicyOrange through his ebay store.
I will start with the bad news. Details can be found elsewhere but here is my summary
VCTS was installed for emissions during cold start; it warms the first catalytic converter quicker. This provides the P5 with the ULEV rating. MP3 ECU eliminates this function.
Now for the good news.
As tweety2300 said, it is easily the best single performance mod for N/A P5. Coupled with synthetic trans/motor oil, the removal of my roof rack, Simota CAI and the MP3 ECU, my top end power and efficiency is amazing. The car has passing power as the acceleration from 60 MPH to 80 MPH is phenomenal compared to stock. Acceleration on hills is an actual thing now. I have also brought my cruising RPM lower, so now 55 MPH is below 3000 RPM, though I attribute that to synthetic oils more than anything (Redline FTW).
The only drawback is that the power is mostly above ~2700 RPM. I am looking for mods with low end power. I need a cat-back exhaust and I really want a header.
Other than performance, it makes the car easier to drive in some ways. The start up chatter (the loud slapping sound on start up) is completely eliminated. The car still has a typical cold-start procedure of ~1500 RPM idle, then dropping to ~1000 then lowering to normal, which for me is just under the 750 hash mark. I wait until it goes down to ~1000 RPM before rolling out.
Shifting is better (also paired with redline trans oil) and acceleration is smoother because the intake is not suddenly changing air flows.
TL;DR: Loses ULEV, changes low RPM dynamics especially during cold start; gains smoother acceleration all around, high speed and RPM performance, becomes a beast on highways. Highly recommended, would buy again.