The bigger front sprocket will give you a higher top speed like you mention but it will be at the expense of some low end (might require a little more pedaling at starts or up hills). You should also check out what rear sprocket you have since Sachs came stock with either a 44 or a 40 tooth in the back. Just play around with the combo until you get the compromise between high and low end that you want.
One thing to keep in mind is that if you go to a 13 tooth front, you will have to grind away a little bit of your engine cases to make room for the bigger sprocket. I have done it for a 13 tooth but I would be a little wary about going larger since the metal is pretty thing there and you might end up making a whole and leaking tranny fluid. Your chain would end up being very well lubricated but leaving a trail of type F behind you might affect your handling like a James Bond type oil slick.
Going 35 up hill is pretty good for a stock 505/1a. Sachs aren't known to be speed demons (but they can be - see Nick Klaben's incredible G3) but stock they are usually rediculously reliable.