Thanks for the input, it's good to hear another person's opinion, do you have any pictures or the master cylinder /servo fitted and pictures of how you strengthened the bulkhead at all? Do you know what mercedes the servo was from?
I can take pictures of the servo, but if I remember correctly it was a Merc 190 booster, I found it on the internet that it accepts girling master cylinder of the M5 which I already had. Both of them are aluminium so saves a good amount of weight especially with the ABS removed.
Anyway I think what You need to consider is this: all the power you exert on the brake pedal need to go to the brake calipers via the leveraged hydraulic system. So all the flex you have between these two points acts as a power loss. My first post was not exactly on point because not only the bulkhead flexes (it flexes less) but the standard brake booster holder flexes a lot more which is made out of ridiculous thin sheets and it is a horrible design, as we know it. Also the unfortunate placing of the steering rack makes it hard to place it close to the bulkhead.
You can check for yourself, someone pushes the brake in your car and it does the same:
In an MK3 it moves a minimum of 10mm. Obviously seperate pedal boxes get rid of these effects, but it is another story.
The holder I posted earlier has a thick baseplate of 5mm and a 4th bolt that connects it to the bulkhead (despite the limited space), this is it. And you can see finished holder, the 190 servo and the finished assembly:
It was pretty hard to get the angle right as the standard one is also tilted upwards as I wrote earlier. (My motronic wiring is a mess right now on the picture I know, it is obviously not a showcar
)
So in my opinion maybe it is better to get a bigger standard vauxhall master cyl and create a proper, rigid holder that connects multiple ways to the bulkhead and therefore it is a lot less prone to flex as the standard one.
It worked out quite well for me. Maybe later I will go for a fully machined aluminum holder and some more refinement on the bulkhead.
These are my 2 cents