Hi, :)
Just a quick answer so you did not feel that your question was not being addressed. Yes, 500K volume for humbuckers have by now proved notably clearer / brighter / more '3D' sounding than 300K.
I have a 70s Gibson S.G. that had 300K pots in - and I only heard it properly for the first time when I replaced them with 500K pots . The 300k loads down and therefore dampens the response of a Humbucker quite noticably to my ears.
Only recently I gather there has been a flurry of activity on here with 'Warpig' owner(s) experimenting with 1 Meg or even higher value pots.
I remember some time ago ( over on the Telecaster discussion forum ) some 'ol boy - insisting on putting 1meg pots in his Telecasters !!! Crikey, with them sounding 'hard' with the more usual 250K pots, this guy must have been shattering beer glasses with the sound of the bridge pickup !
I mention this , because it just goes to show that higher and higher values do indeed 'un-inhibit' the output and 'soundstage' of any pickup - but nice tone combined with maximum clarity deserves a little calculation - or more practically experimentation for best results.
I have only the low output B.K.P Humbuckers and 'P90' s and so for me 500K is perfect. I hope the ladies and gents who own the ' truly monsterous' end of the B.K.P. range add an answer soon ! :)