i've heard that the remapped mjet's easily go off the clock with no trouble at all so i'd imagine easily 140 on the clock mate. I mean the tyres (if you've stuck with the manufacturer recommended one's) are rated to 150/155mph so there's no danger there and the 1.9 is a very strong engine so i can't imagine you causing any trouble...certainly not trying to test the top speed for a short period of time.
I've had my virtually standard starjet to 130 (all be it down hill and clock speed) and it was fine. Felt just as civilised as it does at 70/80 other than the fact that it was pretty high up the revs!!
also if the tyres say 150/155 then there will be a huge safety buffer there aswell, wouldnt want to try it but ill bet the tyres are good for easily 200.
Generally speaking in terms of safety everything is built to the rule of 3. e.g. if a lift says it takes 10 people, it should take 30 before breaking, if a winch says max load 1 ton, it will take 3 tons. and so on and so on.