After latest BIOS updates - current boot time is around 40 seconds from button press to Windows login screen. System also seems to wake from sleep successfully. The startup tests definitely seem much faster though - I'll keep monitoring it and see how it goes.
BTW - does anyone know how long it takes for your PC to go into the different sleep modes? I believe the issue I was previously encountering was with one of the specific sleep modes, although I'm not sure which one. If the PC was recently active, but is now inactive, it can be easily restarted. If it's been inactive for a longer period (e.g. several hours / overnight), then it wasn't able to start without a full restart.