I've never owned a SF2 but do they really take that much abuse? In a home environment will you need to worry?

That being said one way to strengthen the part would be to make the weak areas thicker somehow. I've actually glued clear pop bottle plastic to thicken an area of a ramp that was already cracked using super glue, it's help up pretty well. An extreme method might be to carbon fibre re-enforce it but that's probably overkill. Unfortunately I don't know of any product that you can coat the plastic in to make them stronger.

I've wrapped aluminum foil tape around cracked subways. Inside and out. Works great.

I'm guessing that you have problems around the VUKs and perhaps the entrance to the upper left subway (Dahlism). These are the holes that can take direct shots from the flipper. The entrance to the upper right subway is the spinning kick and the ball simply drops in there with little added energy. I'd be surprise if the subway up there was broken.

Truth is as Prock suggests, in a home environment, you are probably not going to ever have issues with new subways as long as you own the machine. Gottlieb used pretty thick plastics and they held up reasonably well. If they were a real problem, you wouldn't have been able to buy them from Steve as he would have sold out a long time ago.

Having said that, in addition to the other suggestions, you can also try using some dead drop foam but you have to make sure it doesn't interfere with ball travel or mechanisms:

http://www.pinbits.com/index.php?main_page=product_info&products_id=131

viperrwk

I'd pick up some epoxy resin and fiberglass cloth, scuff the underside for some adhesion and start lamming it up.
the epoxyglass will stick better than polyester resin (normal fiberglass), its lighter weight, and stays a little flexible so it doesnt crack up like regular fiberglass. a pack of fiberglass cloth from hardware or auto supply will be a little thicker than what I'd usually use for boatbuilding but its a convenient grab and more than enough to do the job. 4 layers of it 2 layers per day (lay up two, next day sand and 2 more layers), day 3 oughtta be ready to install and a whole lot tougher than stock.. or if you dont think so, do another day adding 2 more layers to that. it'll back up the plastic from behind very solidly once done.