I agree with Greg and Kirk.
There may be a car out there that maybe using the "ITC" model chassis with the "ITA" model drive train might provide some advantage other than weight, but I can't think of one.
Suspension geometry maybe? This is typically very doubtful as long as you are staying within a generation run of a car, but there may be something out there.
The benefits however would be great. Especially in terms of cost and availability.
Take the good old 88-91 CRX Si for example. Good ones are getting harder and harder to find, so if I'm racing one now and I completely bend it, I have to find another Si in which to swap over my unbroken stuff.
Si's aren't easy to find these days, and if you find a good one, the owner wants to be compensated dearly for his prize.
What would be nice would be to have the freedom to use the CRX HF chassis. There are thousands of them out there for pennies (there was one on ebay for $60 last month. Thats sixty (not 600) dollars). The ONLY difference between this chassis and the Si is the lack of a sunroof (which solves another problem cheaply and safely... plugging the sunroof hole).
As long as the minimum weight is unchanged, what difference does it make?
In terms of Hondas, theres nothing out there that provides any advantage other than weight by using alternate trim level chassis. The differences were always the drive train, sunroof, power windows, and the interior trim.
When we wrote the Honda Challenge rules we allowed this. It just makes things simpler and easier on the competitors. The caveat we made (and yes, I know its a gray one) in the rules was that the use of an alternate chassis could not provide a performance advantage beyond what was already allowed in the rules in terms of modifications.
In other terms, You could use a 1991 CRX HF chassis with a CRX Si drivetrain and run it in the CRX Si class. The only difference in the chassis is the sunroof, and the rules allow you to remove and re-skin the sunroof in the Si. Therefore the HF chassis provides no performance advantage, its just cheaper and easier to deal with.
I hope that makes sense. It makes sense in my head.