It's easy enough to point to the systems that have been pushed to their limits. It's usually the systems that were on the market the longest, or at least competed in their respective generations the longest. Atari 2600, Intellivision, C-64, Apple II, NES, Genesis, SNES, PlayStation, etc. It's hard to think of very many systems with very short commercial lifespans (two to three years or less) that we could conclusively prove were topped out.
I don't know that I'd say that about either the NES or the SNES. So many games had add-on chips in the cartridge. The SNES had the DSP family, the SA1, SuperFX, etc. and the NES has a legion of mappers that did all sorts of stuff, and it's hard to tell where the system stopped and the add-on chips began. Two potential exceptions are Final Fantasy III and Chrono Trigger, neither of which used the chips but looked gorgeous. They were exceptions, though, from what I can tell.
The Genesis has two games total with extra chips (from its original run, anyway): Super SF2 and Virtua Racing. Yes there was the 32X and Sega CD, but those add-ons incurred up-front hardware costs and were marketed as distinct platforms. The PlayStation had zero add-ons, so developers were stuck with what they had. Not sure about the pre-NES computers (although certainly they had upgrades available) or the VCS, but they may fall into this category, too.
Edited by derFunkenstein, Fri Oct 27, 2017 7:40 AM.