Having spent a week there, I can testify that there are enough roads of the right sort for a short HPC event (e.g. a long weekend or a 3-5 day trip). For people living in the south, it's no more effort to get to the IoM than Scotland.
Dave's correct that about the size of the island and the mountain course is only 37 miles long. However, I found enough interesting roads to enjoy riding all day for 5 days. It'd be worth HPC taking a look and perhaps doing an exploratory visit.
The nature of the IoM means rides had a different character: shorter runs, but higher speeds. Due to the speeds involved, however, I found it beneficial they were shorter because of the concentration demanded at speed. It's very possible to do 130-150mph on some of the Island's roads, which would be dangerous to sustain for long periods without a break.
Overtaking opportunities were never a problem! And where else in Britain can you legally overtake a police car at over 130mph (as happened during my week)?
One thing I would say is that the right instruction is cucial for safety. It's one thing doing an HPC drive on the mainland, perhaps pinching a few MPH in the NSL, but quite another to sustain Isle of Man speeds for a week without human error creeping in. That's what the
Alpine TT course does so well. We were carefully instructed, with lots of breaks for ice cream or tea, gently building up speed, using police Class 1 instructors with vast experience of coaching civilians at speed. The whole thing was very enjoyable and very safe, which takes a lot of doing.
It was also a brilliant holiday, with fun tourist stuff carefully mixed into the week to ensure it felt like a holiday too. Keeping us relaxed doubtless helped safety, by giving us a chance to recharge and enjoy the island.