We run 21.5 no timing, 17.5 no timing and mod. Good spread of guys through the field.
21.5 is cheap to get into, you can get the excellent speed passion cirtix 21.5 combo for around $130 now, and properly geared beats most 540 silvercans. No one runs 540 silvercans at our track anymore in that class.
The new breed of 17.5 motors with timing on the motor provides enough adjustment in the stock class and nice learning curve up from 21.5.
I think the hobbywing 13.5 motor/speedy combo is ok for mini, but compared to the price of the justock speedy and v3 speed passion motors its quality lacks a bit.
I personally think there are too many options, but unfortunatly with the amount of motors and combos we are blessed/cursed with choice and need to make some hard decisions.
I think club race days should be limited to three classes, with larger meets over a few days open to 5 or six classes. Seems to work reasonably well for our local club days and the bigger interclub events.
I think if there is too much confusion, and rules changing to often, people will leave the sport, as it costs far too much as it is, and if we have to throw gear away at the end of a season as it is not legal to run anymore people will leave and go drifting or something cheaper