I don't actually believe you. I believe my own ears. Think again about what I actually said.
What? If it's not laptop speaker quality, it's not the target hardware T10 had in mind when mixing and mastering, and when showing off to higher management.
Ah, so you really didn't understand what I said, just as I thought. Headphones have the potential to give the best signal to noise ratio overall, and my headphones in particular are actually designed to highlight flaws in recordings, so I think they are more than adequate for assessing quality.
They optimised their game to work with low end hardware
only. Re-read what I wrote, and read the article I linked to, please:
http://productionadvice.co.uk/mastering-for-earbuds/
Volume generally does, yes. There's an old joke in the recording industry that a 3 dB level difference corresponds to a doubling of the audio quality. This is why absolute level differences are carefully controlled in comparing different hardware or different media etc.
GT5 Prologue sounded breathtaking at high volumes (on actual speakers, filling a room, or headphones), especially with the better sounding cars. That's partly because of the way the sound can "breathe" on the high dynamic range settings (Small or Large Theatre), and the directional audio engine PD developed.