I bet you can go pretty high. Easy way to tell, is to just unbolt the bottom of the current block, take all the wire ties off of the cables etc, pull all the slack out, hold the handlebars up and have someone measure the gap. Bet you can get to 5" easy. You can buy expensive risers, or inexpensive (like powermad). Best bet though is to get one that pivots, so that you can change your riding stance to your desired set up.
