Well, lets not get too wrapped up in 'less real'...
I would say that there is an immediacy if you can say "yes, my belief IS real" or at least define some things that happen/are true in game that relate to it (maybe they have some other explanation, this uncertainty might be useful). Anyway, remember, players don't get authority to just "make my wishes come true", at best they might stipulate things that engage their agenda. The GM will play a key role in determining what exactly it means. It could be that "Yes, indeed Corellon is real, and he's also got a chip on his shoulder for humans, OOPS!"