the fabric on your current fly is probably between 2 and 2.5 oz/sq. yd.
So the MOST you would save with silnylon would be 1 oz/sq. yd., as it is about 1.3 to 1.4 net wt. you could save more with cuben, depending on whihc version of cuben you use, but it will be much harder to get a good fit that yields a tight pitch, as the cuben does not stretch appreciably and is thus much less forgiving in terms of fitting a fly to an existing tent.
Acording to the Fibraplex webiste, you can save about 6 oz by going to carbon poles.