After this discussion, I made several calls with heavier latex (.004) than I typically would use,...and really stretched the latex layers out, as well as adding some serious backstretch on some of them. Again, not using any tension measuring devices, I just guessed,...but I would guess I was stretching the material a half inch or thereabouts. How that translates to metered tension, I don't know,...but it is definitely more tension that I typically use with thinner material.
The results are very encouraging. Really getting some good sounds out of the calls. For anybody that hasn't tried stretching heavier latex tighter than what you might think,...I recommend giving it a go.