Steve, I haven't heard anything about valve size in the discussions. I actually do think that's a place to look, when choosing a car, BUT, even it is full of exceptions.

The CRB has just recently drawn it's line in the sand at displacement. But recent history shows inconsistent application of even just that:
-The BMW 528e recommendation we made was to place it in ITB. We have great intel on that engine, and we know it can't hit ITA weight. The CRB rejected the recommendation, stating that the engine displacement was too big for ITB. So, it will have to race overweight in ITA. What's the over/under on the letter from that owner requesting lexan windows?
-Conversely, they rejected the weight of the MR2 variants, and assigned them higher weights, even though their 1600cc size is nothing out of the norm for the class.

Even combining displacement and valve size won't, I think, net you a more accurate gauge of gains in IT trim, because it ignores/assumes all cams, intakes, and heads are equally efficient.

While it's certainly not dead accurate, it is felt that for IT, that stock hp actually reflects those variables, and is a better place to start.