"Would Lewis be beating Seb if both were at Red Bull?"
The short answer is UNEQUIVOCALLY YES.
The long answer is most teams adopt a 'split strategy' i.e. sign two drivers with different driving styles, run different strategies with tyre choices etc to cover most eventualities and maximise their 'success rate'.
I think Vettel and Hamilton have quite similar driving styles and although both of them at Red Bull sounds like an interesting prospect, it will probably never come to be.
Nonetheless if Hamilton was at RBR in a car designed by Newey, he would have THE best car on the grid. Compared to Vettel, I'd say he is marginally more aggressive in seizing opportunities to overtake but he'd be doing very little of that since he'd be qualifying on pole. Both drivers are skilled in wet conditions but Hamilton has a slight edge on Vettel, as the following video demonstrates...
Japanese Grand Prix, 2007
So their relative performance would come down to their Race Starts and Tyre Management. Tyre Management is considered by most as Hamilton's weakness but his race starts are fairly good.
So with all this considered, I'd say Lewis would be beating Sebastian but the gap wouldn't be as big as the chasm between Vettel and Webber at the moment!