I have a servo, late GT6 mk3 had one as standard, a Girling. It's actually an MOT fail to remove one if it was fitted as standard and before you say "How would they know? The MOT man told me that. Bizarrely it has to be there but it doesn't have to work, there's no proscribed test for it. Which was fortunate for me as mine never worked, the car was 5 years old when I bought it and the servo didn't work. Only in recent times have I fitted a Lockheed which does work.
As Colin says It doesn't make any difference to braking you just have to push harder OK for "Thighs like tree trunks" Lindsay to say, but I'm 75 you know . I put the Lockheed on precisely to make the braking more like a modern. I've had a servo fail in a modern and you REALLY can't stamp hard enough to get the same effect.
The servo sits between the Master cylinder and the slave cylinders introducing an interim cylinder which uses the vacuum to "amplify" the pressure to the slaves. I can see how disconnecting the vacuum might not give you the same as if you'd completely bypassed the servo. However, running around for 30 years with the non working Girling I can't say it was any worse than when I put the Lockheed on and initially it didn't work.
On the mk3 the servo only works on the front wheels, on previous GT6s the servo was an option and worked on all 4 wheels. Odd! However once having got "into it" while fixing the Lockheed I tried it on all 4 wheels and very quickly put it back to front only. They had upgraded the rear brakes and putting the servo on all 4 made TOO much braking at the back.
The Lockheed's problem was it had been poorly assembled in China. When I took it apart and cleaned it up I got it going, Nirvana! I wouldn't be without it, if it fails it will get fixed very quickly.
Doug