From what i've gleaned in the past few days, 48t seems to be the limit for 'MTB' style front derailleurs.
Since you have it there already, however, you might want to try it and see. I suspect (but this is just idle speculation) that if you are using indexed front shifting, it might not work, but if you are using bar-end shifters or some other friction-ish option, there might be a chance.
I've noticed that NOS road front derailleurs are pretty cheap on eBay. I got a brand new Ultegra triple for about $25 shipped, and the only reason i went for a brand new Ultegra model (instead of a used 105 unit or something) is because i'm vain and insecure and live vicariously through the envious looks my bike gets
