Restricts really nicely.
My wife has a 1998 GPZ restricted using £40 carb washers from eBay.
I would say that the bike restricts really nicely. The low-end power delivery is essentially unchanged and the bike pulls fantastically well low-down in the rev range. The bike takes a while to rev when restricted but does have decent pull at the top end if you want to push it (though you really don't need to).
The engine is fantastically smooth and it as happy at 8k rpm as it is at 4k rpm, it pulls good and hard from 4k up to 7k.
Within (and slightly over) the speed limit the restricted GPZ rides really well. I recommend it
I'll let you make your own mind up on that price, but FWIW I think it's certainly at the high-end price-wise.
Edit: If you've only ridden a 125 you'll think it's an absolute rocket.