I've only raced with ThundersportGB once, a couple of years ago, so take my view in that knowlegde.
short version: they are brand-focused, not racer focused; although they appear to cater for all sorts, minor classes (where we in 400s can race) are neglected.
longer version:
I don't rate them much and creating yet another class is typical of their approach. They spread their brand very thinly - it is all about sounding like they accommodate all sorts of classes and races, about seeming to be the club racer's friend and about getting TV exposure. I experienced the reality a bit differently.
The main practical things I didn't like:
1. Small classes - note, not small grids (although many are), but small classes.
2. Mixed grids. Some classes have only 2 or 3 entrants, and they have to be put somewhere.
3. Because of TV coverage, they keep all racers in a parc ferme after the race, to film the podium finishers, interview them, and have an audience. Anyone who has raced a ZXR400 will understand the issue with killing the bike straight after a race and then trying to start it again 5 or 10 minutes later.
To illustrate points 1 and 2 above, see for example
these latest results. Go to the GP2/GP3 results - F400 riders qualify for their GP2 class. Race 6 had 13 finishers across what looks like 3 classes: GP2 (6 riders), GP3 (2 riders) and minitwins (5 riders).
You'll see there is good support for some of their classes - CB500 for example. However, for the minor classes, expect to share the grid with all-sorts and know that you will almost definitely have a top 5 finish!
Interestingly, their F600 doesn't seem to be well supported. Most clubs have several full grids of various 600 forms (stock, sport, formula, steelies, etc). Could this be a sign that riders in the best supported class in the country doesn't rate TSGB either?
Give me Bemsee or DP or NG any day. The guys at DP had a moan about having some old big bikes on the grid at Cadwell the other week, but at least we had more than 20 F400s on the grid.
Would be good to hear the views of people who still race with TSGB though; maybe things aren't as they appear.