My 556 Patrol is with my gunsmith right now being converted to an SBR (Form 1).  He hasn't ever pulled the barrel off a Sig 556, so he found someone with a lot of experience doing it, and purchased an armorer's block from him.  The guy (I didn't catch his name) had some disconcerting things to say about the removal of a 556 barrel.  

1) The barrel is threaded into the receiver (this is good)
2) There is 400lbs of torque holding it in place!
3) Even with the armorer's block, there is a chance the pin and pin holes in the receiver will bend/break before the barrel will come loose!  
4) He has seen just as many break as not!
5) The XI barrel is held in place with just ~50lbs of torque.

I am NOT happy about this!  The barrel can't be cut/threaded unless it is removed from the receiver.  

Has anyone had issues with removing the barrel?  Any horror stories!?  This upper is now a SBR'd receiver, and I really don't want to break it....

if it worries you that much I'd ask for it back and send it to the pro's at ADCO.

This. ADCO has the experience tools and excellent reputation

Assuming he has the correct receiver vise and barrel wrench, it is not difficult to remove and install a 556 or Swiss 55x barrel. The receiver vice protects the receiver. The only way I can see that you could damage anything is improper/inadequate tools or serious operator error. I did a number of removals/reinstalls of barrels from demilled receiver front ends before I tackled a complete barreled upper. Once I felt comfortable with the process and in using the proper armorer's tools, only then did I work on complete rifles. It really isn't difficult, but if you do it incorrectly you could very easily destroy the receiver. The bottom line - send your rifle to someone who knows what they are doing. Experience matters.

Feel free to email me and I will get you the contact info for a good 55x smith