Just remember one thing about demos: as impressive as they can look they're rarely a good demonstration of what the *realistic* capabilities of the machine actually are; definitely watch and take careful note of the "how they did it" video before getting too excited about what kind of amazeballs video games the IIgs might be capable of. These demos are basically just amazing exercises in coming up with ways to shovel pre-calculated data onto the screen according to an excruciatingly rigid script, while a real video game needs to do a lot more than that.
(Also note that these demos target an 8mhz accelerator, almost three times as fast as the stock machine. I do kind of want to download it and see what it does at 2.8mhz, though, given their talk about the limiting factor for a lot of it is the 1mhz video RAM bus.)
Having lived through owning an 4.77mhz IBM PC for years I feel like I'm pretty sure I have a good idea of what you can reasonably expect from a game written for it, but if I *didn't* know seeing these demos might give me some unrealistic ideas: