Rear Derailleur - Short to long cage

Not answering the question I know, but SJS cycles are fantastic for those hard to source parts.

You can buy a long arm for the derailier and swap it out, not sure where from though as the bike shop did mine.

These look just like wolftooth and are cheaper, as Welsh boy wrote:

https://www.ebay.de/itm/Bike-Rear-Derailleur-Hanger-Extender-Road-Bicycl...

I've just bought a Wolf Tooth Road Link to faciliate a larger cassette.  It's pretty expensive for what it is - a drop hanger for derailleur -  but it's cheaper than a new derailleur and you don't have to break your chain (which in old money means a new one).  They're GBP 23 on AMZN Prime (an outlet I rarely use for bike parts,  but few bike shops stock them - not even mine, which is Condor).  

Good answer from Woldsman, another thing is what's the largest sprocket you're aiming to use, the 6700 SS I know for definite had a Shimano stated 30T max. IF the 5700SS has same I reckon you'd be able to do 32T with the usual B tension screw done right in. Obvs if you're going bigger then that's probably not doable.

currently I run an Ultegra 6800 SS with a 32T and it was only designed for a 28T

Any of the 7/8/9 speed MTB mechs would work if your XT isn't 9 speed era.

Assuming your XT is 9 speed you can swap the entire mech to replace the 10 speed 105 rear derailleur. Do be aware that some XT models (eg M772) don’t have a barrel adjuster, so you may need to fit an inline  one (eg Jagwire).  

Note: you cannot simply fit a 10 speed XT to a bike that previously ran a road 10 speed system such as 105 as the shift ratio is different.