What kind of gun are you talking about?
Just on face value though I would not immediately dismiss the guide rod simply because it is plastic. Half the guns on the market are half plastic these days and you rarely hear about polymer frames cracking. It does happen sometimes but my point is that polymer technology has come a long way. There are polymers out there that have higher abrasion resistance than steel and have melting points way above what most polymers can stand. Now of coarse I have no way of knowing what kind of polymer your guide rod is made of but there are polymers out there that are more than up to the task of serving as a guide rod and I would be willing to think that the good engineers at Sig chose a good one.
If you have further interest in the properties of todays engineering polymers you can wikipedia a couple such as http://en.wikipedia.org/wiki/Ultra_h...t_polyethylene .
Or go to www.mcmaster.com and look up all the polymers they sell. FYI McMaster-Carr is an industrial supply company if you have never heard of them. They basically stock just about all the raw materials you could think of to build anything with. Great company.