Do it, they're great.
I've done Brands GP circuit a few times on my ThunderAce, as traffic light Grand Prix's in London (and up Denmark Hill in particular..) were getting a bit scary.
The GP circuit costs more money, but is beautiful around the middle section, and has the two near top speed...
I think you're right..
http://blog.leatherup.com/wp-content/uploads/2012/06/Biker-Girl-Motorcycle-Girl-Playboy-Playmate-Raquel-Pomplun-on-a-Victotry-Motorcycle-Wearing-Black-Leather-Jacket.jpg
Well you can still do the fast track 5 day course i think, and that will let you go above 125, and that has to be recommended - i shudder to think back to a time when my little RG125 couldn't get me out of an idiots way with a twist of the throttle; a 500/600 will let you do that.
As above...