Using your 10 speed deore,slx,xt,xtr, rear dérailleur for 11 speed is possible, but it wont be as smooth.
but of course to compare this you have to have exact same setup, which I had here.
And of course The clutch tension must be properly adjusted & all other adjustments, I set mine up by decreasing tension until it the shifting performance was acceptable.
I recently rode the bike with SLX M675 few days and then the xt m8000 bike after wards and the performance is definitely noticeable.
Even on 11-42T cassettes with stock links/without goat link, the difference was noticeable.
the difference was most noticeable in the lower gears/bigger cogs.
If went up and down much faster, and made less noise.
I used KMC 11X SL Silver on both setups.
Is it absolutely necessary to upgrade? absolutely not,but on rougher trail it’s nice to have better shifting.