Chris/Monty,
IMHO the discussions are paying off now.
I know I'll sound like a stuck record but I agree that a 583 with limited mods (Dynojet/K&N/can + std everything else) will be close to a 620 up to a point and will provide a worthwhile grid and racing.
Then those with 620/675/minitwins/money can have their own battles and probably a natural cap on mods/cost will evolve over a couple of years.
Will the ACU/New Era sanction two classes in the same series?
If your two class plan is a goer the chequebook will be dusted off .......
Paul