LFH:
The Mk2/Mk3 seats had different limits depending on the cartridge fit, which gave varying seat ejection speeds.
You are right about the Vampire: I flew the T11 in training, and it had a 60ft/sec gun, with limits of 200ft/120kt. The story we were given was that the Vampire structure was not strong enough for the 80ft/sec gun. True or not, I don't know - but your comment on retrofit with other seats seems to contradict that.
Later I flew Hunter 6/7/8/9/10 with Mk2 or 3 seats in the single seaters, Mk4 in the 2-seaters, and all had the 80ft/sec gun with limits of 90kt ground level.
AFAIK, all UK Hunters had that same capability.
I have no direct knowledge of the Swiss Hunters, but expect that they had the same seat fit as the UK aircraft.
KP