Just downloaded Under my Thumb from Cedar Falls (thanks!) and, as ever when I listen to an 81 boot, I cannot believe how different it sounds from the Still Life version. The sound is radically different - it has its charm, a little sloppy and ill defined as it is - from the tough, clear, sharp sound of the official release.
As a matter of fact, the 81-82 tour is the only tour from which I favour the official release over the boots.
Now, I remember a post here (perhaps Mathijs?) saying that Still Life was not overdubbed, the songs were only edited and shortened. Am I right, and is it true? Is it then just the mix that does the huge difference?

I'm shure there are at least vocal overdubs. Just compare "Let's Spend The Night Together" from "Hapton 81" (which to my knowledge appears on "Still Live") with the released version. And - all the more instructive - compare both versions of "Time is On My Side". They wouldn't let Keith's backing vocals unchanged.

...and Keith's vocals on Let Me Go. It didn't sound that clean on the concert.

Many songs for the movie Let's Spend The Night Together were edited/shortened.
Of course there are overdubs on Still Life. But then again, the song "Let's Spend The Night Together" doesn't sound very 'reworked'.

I don't have problem with overdubbing cock ups and bum notes on live releases. They're no problem in the context of a live concert but do get annoying on repeated listening. Better to fix it in my opinion.

At the time Mick said they only overdubbed some vocals, mainly due to loss of mic. contact.