I'm wondering if it would be worth it to buy the XB1/XB360 backward compatible version just for practice to get the levels down? I own it for pc but my girlfriend uses it for LoL and other stuff so I can't play it all the time.
It should be fine, from what i know there isn't any differences between the pc version and the console version. so if you do buy it on xbox you should be fine to practice levels :)
What azorokazu said is true, though remember that the absolutely awesome original musics you can have on PC aren't on Xbox versions. (which still doesn't change anything for practicing speedrunning, right)
load times - if you're running on xbox 360 you're going to have a serious disadvantage. i'm unsure about how xbox one loads stack up, but if you want to throw together a comparison video, i'd love to see it! a lot of us have been curious and unsure about how this game fares on current-gen consoles. good luck with fast!
XBox performs perfectly bar the load times mentioned by warm ham for meat boy:
Xbox is perfect for practising any % since even if load times are different the play should react the exact same afaik
XBox 360 can't skip cutscenes so you are at a disadvantage for dark ending and 106%. Not sure if xbone can skip cutscenes
You don't have naija, josef or runman so you are at a disadvantage in 106% too even though spelunky might help a tiny but but I suspect very little.
Finally you won't have the same speedrun potential on ILs since aside from spelunky and maybe ninja(not sure how much he does) all the characters and more are in pc version and missing naija is a big thing here.