I went ahead and did this a few weeks ago. Needed to do it since I installed a 4 wheel BBK (I had the base C2 brakes which are not good enough for me on track.) Larger calipers mean longer pedal travel with the original MC so the GT3 MC it was. Went pretty well. I did it a bit differently, though, based on other accounts. I did not drain the system. I did it by

• removed the plastic cladding/housing from around the MC and disc changer
• covered as much as possible with towels, mainly under the MC. Lots and lots of towels.
• drained out fluid until the reservoir was about 1/2 full.
• unbolted the MC from the booster
• "slid" MC out of brake booster housing (this was the hardest part). There was just enough room to get it out with all of the plumbing still attached. Secured it out of the way temporarily.
• bolted the new MC to the booster.
• performed a rudimentary bench bleed of the new MC.
• Disconnected the front MC line from the old MC and connected it to the new MC, placing a plug from the new MC into the now open old MC line hole.
• Disconnected the rear MC line from the old MC and connected it to the new MC, placing a plug from the new MC into the now open old MC line hole.
• Pried off the reservoir from the old MC and quickly pushed it onto the new MC. This was the messiest part.
• Bled the system normally except made sure to work the brake pedal in the car half way a few times during the bleeding.

It was messy but the towels caught everything. I did not need to bleed the clutch (woo!!). It turned out great. As far as I can tell, there's no air. There's some dead pedal prior to engagement but then it's like hitting a wall -- which is what I want. I think I need to adjust the pedal (?). I'm not 100% sure about that, though. Everything seems fine otherwise.