PS: Here is an estimate on your multiclassing: 2000 in ITA, 2350 in ITB and 2760 in ITC. Think you can get to anything other than ITB? Like I said before, DC works for tweeners, probably not for cars classed properly.
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So right now we have a process where the ITAC generates number like this and then makes a value judgment on which class the car fits in. In the above example it looks like an easy decision. Maybe you could do a very expensive build and run A, or run very heavy in C, but B looks right.

With tweeners it is not such an easy call. Take the MR2, the ITAC has their best call on which class drivers want to race the car in. As with any close decision half the club will not be happy, so if the car works in either class why not let the members decide where to race it.

I think the double dipper argument is a red herring. It is not a reason to have DCs but it is not a reason not to have them.