Sprocket changes are what you want to do. The change from 13/48 to 15/46 would be huge - maybe too much. I would try a 14 tooth front sprocket first before changing anything else. On some bikes, to run a 15 front you have to remove the guard that protects the cases in the event of the chain derailing.
You may have to alter your chain length if you are making big changes to the sprockets.
BTW, the tires that fit the 19" rear rims are generally lower profile than 18" tires, so the overall diameter is similar, and you would get very little if any change in your top speed.