NicD,
Sorry if I've upset you. This forum is supposed to be fun for everybody. Let's both try a little harder to keep it that way.
I'm sure you're a great guy and it's always easier to have these discussions in person than it is over a forum. The funny thing is, I think we agree and we're simply getting hung up on semantics.
I believe I totally understand your point that without the air filter change my results would have been different. In other words the air filter makes a difference on what's achievable with a tune, and I absolutely agree with this point.
My point is, given that the air filter does make a difference and the Z06 uses a different air filter (and air box, exhaust, water pump, OS, etc.), the tune-only gains on a Z06 likely will not match the ZL1 tune-only gains.
You're also saying the engines are effectively the same, so therefore the expected gains from a tune-only should be about equal. However, given the previous point, either the air filter, exhaust, etc. make a difference on what's achievable with a tune or they don't. I don't see how one result can be invalid because of an air filter swap, but a result with a different car and air filter, exhaust, etc., provides an accurate expectation of gains.
In a roundabout way though, we've both basically made the same point that outside influences, like air filters, make a difference on what's achievable. Nothing groundbreaking there.
Your experience leads you in one direction because you've commonly seen lower gain numbers. My experience leads me in the other direction because with only a stock replacement air filter and a tune I've seen numbers far exceeding the 35 rwhp mark.
It doesn't seem that either of us know the exact tune-only gains that are achievable on a ZL1. We were both just trying to help out the OP, and it probably just boils down to a subjective opinion on what we each feel is a safe tune.
No big deal. Let's move on.