Lots of lightweights, welters and middles have made the transition ok. Oscar is one. But that's not really relevant anyway. To take on Shane is what made me put Floyd to #1
How? Like you said "Ain't nobody beat him so he's still #1" Floyd hasn't beat pac, so he's not on top
Pac never beat Floyd. But besides all of that, P4P is an almost purely subjective ranking, so people essentially choose it based on who they think looks like the best fighter to them.
Oscar De La Hoya is 5'10 and started his career at Super Featherweight. Non-comparable. My point is abundantly clear, Marquez had no business fighting at 147lbs and for people to hold it in the same regard as Manny's victory over him at 130lbs is absolute lunacy.
And Floyd dominated a Marquez who was coming off back to back KO wins...and besides all of that it was clear that the fight was a tuneup.