My understanding of musicians, practically all of them, and of all genre of music, is that the performance you hear on the CD is always faster than what they actually play in the studio.
No, that's obviously not true - there are plenty of live recordings by all sorts of musicians which one can compare with their studio versions and in fact the studio one may often be _slower_ if they're being more careful to avoid bum notes. Studio recordings are sometimes patchworks of lots of painstakingly recorded bits, but not always - MAH is a classic example of someone who does rather few takes (I have that on the authority of one of his recording producers) because he's on top of the notes anyway. And you know what? It's not always worth the bother. I recorded some songs by Alan Bush a few years ago with _horribly_ difficult piano parts, one of which I recorded in little bits over and over (this was just the intro, you understand - wouldn't have done that to the singer!) because I knew from _feel_ I was playing wrong notes. But when I got it back home and edited it, I realised that I couldn't hear anything wrong with most of the longer takes so I saved myself editing time and used those. I mean, if I couldn't hear it (and obviously I know the pieces, I had the score, I've got pretty sharp hearing and I was listening on top quality equipment) Joe Doe was pretty unlikely to.
However, I have also done some very dodgy edits to recordings that have been made in a hurry with not quite enough takes - I've done things like removing wrong notes, adding missed notes, retuning dodgy notes from singers and string players, you name it.
Coming back to the original subject of 'cheating', it's worth remembering that most pianist/composers hardly ever play a piece of their own the same twice - what seemed like a good idea last week may feel all wrong today, so they'll modify it a bit. Is that cheating? And why should anyone care? So, since most of the most difficult pieces are actually by pianist/composers, what's wrong with a little tasteful modification here and there? In all probability the composer would be the last one to care.