Why?
Have you heard of one breaking or somehow degrading performance? Ever?
(This topic comes up a lot on the SIG Forum, where folks think metal is always better than plastic...)
If you shoot IDPA, making the change may disqualify your gun from the Stock Service Pistol class.
Until I see a reason to change, my 85 Combat will continue to use the stock plastic guide. I've probably got 7,000 rounds through it, now, with no evidence of problems.