Taller tires will give you more top end so you will get more speed. If I remember correctly the SX's are shaft-drive so you're kind of limited in that regard, so you'd have to get into the engine. First you need to decide what level of power/speed you're looking for, I would think that it would be pretty difficult and expensive to get 350x/250r results from a SX. So if you want that then you might be better off adding another trike to your collection.
Jason
85 ATC 250R - restored stock other than 18" rears, nerf bars, Honda key switch, 14T front sprocket, and white tank and plastics (except rad shrouds)
87 ATC 200X - restored stock other than 18" rears and nerf bars
84 TRX 200 - rough but complete budget restore for wife and eventually daughters to bum around on
Eton Viper 50cc - oldest daughter's current ride