I have one and play it live. I don't have any clips of channel 2 boosted. IMO the 6505 didn't sound any better above 3-4, just a little louder and crappier. IMO the sweet spot on those amps is right around 3.5 on the post gain.
I've not played my 5150III above 3-4, either, but IMO take everything you don't like about the 6505 and put in MORE of the stuff you do and you have the 5150III. Some people say it isn't as aggressive as the regular 6505/5150's, but for me and my playing style, I think it sounds more aggressive. YMMV
Channel is great IMO but not voiced quite close enough to the third channel for my liking. It definitely can do metal, even without a boost, if you crank the gain. but for me, channel 3 is where it's at.