Enjoy your thread as well. I think you are right about the 30"s being max size with not ill effect on driving performance. however when you go to 31's it seems to really compliment the xj nicely. You may remember I switched from 33's to 31's on my xj shortly before I sold it. Doing it again I would go 31's from the start.
I now have those 31's on my O5 unlimited and I think the BFG km2 is the best all around tire I have ever used, you will like it for sure.
And, I still dig those sawblade wheels... there goes those wheels in my head turning again.... darn MCObray - nothing but trouble...
Best of luck with it all.
John