Why you think the N800 is the problem? Do you have other WiFi client devices that support similar power saving mode and do not cause your router to reset? Unless you have tested that out don't blame the N800...

Regardless, if the router was properly designed no device connected to it should cause it to reset. As you mention you do not see the resets if the N800 is the only device connected, but once the router load increases you start experiencing resets. As a wild guess it seem like the router internal buffer memory gets overflown in these kind of situation, where the router has to store packets for N800 for a while and send them out as a burst... (This is just my speculation... you should ask someone else with better knowledge... I'm no expert!)

Wireless-N specs, as you mention, are still drafts. The N800 is G-device, so any reference to N-specification is moot point. And the MIMO feature you mention applies only for N-devices.

If the N800/N810 works with majority of the routers supporting power saving, I'd still look the fault at your router. If you have a chance to try another router please do so.