Just some input from personal experience:
When employing a multi-class system which includes the SuperGT cars, they are head and shoulders above the rest of the field. It's a good idea to spec cars individually but this can be a lengthy process.
A friend of mine who is quite active in multi-class racing does have a decent set of these regs which make the cars much more competitive with each other. However, as it's so close to race day, perhaps it's a little too late to implement these regs.
Let me know if you're interested in hearing more so I have time to ask him if he'd be willing to share his work.
Somebody like
@Mini Stiggy can vouch for this inequality between the cars as not only have we both raced in an event with similar one-reg-suits-all specs but also he's heavily involved with the FGTA, who themselves have spent a lot of time finding regs to homologate the cars which run in their series.