i am not sure about this either, but i have seen two data sources which didn't make it clear after all:
on www.cpu-world.com, it says the bit before "8"(cache size) indicates an "ice microcode", which i have no idea what it is. blank means present, "n" means not present, while "s" means enhanced.
in an amd datasheet on 486s with 8k write-back cache, it only lists the "sv8b" mode and simply says "s" means enhanced, again i dunno what this "enhanced" means.
unfortunately, i couldn't find datasheets on amd's earlier 486 models.

So far as I know, one of them is marginally more efficient and runs slightly colder, they might have been intended for embedded or mobile systems. They perform identically.

Aha...Googling for "ice microcode" provided some answers. If I understand it correctly, AMD reverse-engineered Intel's 486 to such a detailed level that it also ended up copying some of the internal programming that was built into the Intel 486. This was the "ice microcode" (No idea what it does). Lawsuits were filed, trials were held, and ultimately AMD was forced in early 1995 to redesign their 486DX4 to remove all traces of the infringing "ice microcode". "V8T" variants of the AMD DX4 were from before the redesign, "NV8T" from after.

Now, another question I have that remains unanswered...how, if in any fashion, would this affect the overall performance?

EDIT: Ahh, thanks HighTreason! 😀

Googling provides ICE = "in-circuit emulation", that's what I would have guessed in the context of a CPU. Doesn't matter for normal usage in a mainboard.
If you don't what that is just have a read: http://en.wikipedia.org/wiki/In-circuit_emulator