Personally I've never understood the appeal of indexed classes when there are enough competitors to have viable regular classes. Maybe because when I started 36 plus years ago they hadn't been invented yet

. That said I commonly did my own personal index against the consistently fast classes to see how my performance stacked up against benchmark cars and drivers. It did a good job of either reinforcing what appeared to be a good drive or, conversely, made it clear that I really didn't have anything to be proud of that day.
I used to commonly run the same car in two different groups with different preparation rules. As a result it would be fully prep'd in one group and grossly under prep'd in the other. It was great fun seeing how many, if any, of the fully prep'd cars/drivers I could beat with my under prep'd car. It made me faster. I NEVER (well hardly ever

) assume that the car cannot go faster than I just drove it (since I'm not an alien

) so any "stretch goal" tends to make me try harder.
I purposely did not run TIR with my daily driver tires for two reasons (besides not liking index classes):
If I won it would be no big deal given my three and a half decades of experience. If I "lost" it would be embarassing given my experience.

Much better to run in the normal class (where I had a good excuse for not winning) and try to do the best I could chasing Jim . . . from a distance and maybe give Martyn a closer target to shoot for as he relearns autocrossing after being away from the sport for a very long time.