I'm still using the metro servo that came from Marlin. The master cylinder is a BMW one attached using an adapter plate supplied by Marlin.
It was the clutch master cylinder assembly I changed. It was originally using the BMW one but the mount system on the side of the pedal box was not stiff enough.
I now have a Willwood master cylinder sitting in an ali Willwood mount. It's now bolted to the scuttle which I've also re-enforced.
The pedal box itself did flex so it has two extra steel plates welded to each side to stiffen it up.
Also my pedals run on bearings rather than the plastic stuff that came with the box. Makes everything run completely smoothly
The servo could be a bit more effective as you need to stamp on the brakes pretty hard but you just need to remind yourself you're not in a bit soft easy to use tin top