My Shimano XTR FD is rated for a max. size of 44 - 48T chainring. I installed it anyways on my then-new Ultegra 52-39-30T crankset. It works flawlessly. My advice is to try it regardless of what the specs may say. I have see many other examples of Shimano specs being way off.
I know I'll be pounced on for saying this - but I'm just suggesting people to try. Then publish the results - Yay or Nay.