Hey Neil2704. yes it will go faster in high gear and yes it will go slower in low gear but it's not just a simple put it in and go. you will need another servo and a radio and receiver that has more channels so you can operate the new servo for the tranny.
the 35t will give you more torque than the one you have now and in my opinion would be better. remember the lower the turn number the faster but less torque and the higher the turn number the slower but will have more torque. it would cost less just to get a good 35t than to buy the two speed and new radio. any one that runs these kind of trucks would more than likely recommend a 35t. i would go for the new motor now then later on get the tranny and tx/rx.
just for example if they made a 1t motor it would go super fast but wouldn't climb over a pebble and if they made a 100t motor it would go super slow but probably climb mount everest at a snails pace.