The only reason I could think of that might stop it running a 20v is the distributor. The dissy that comes with it has a 4 tooth pickup and won't fit a 20v, the 20v dissy is completely different as far as ignition pickups are concerned.
Your best bet might be to call Haltech and ask them, it might be configurable or it might need a software upgrade. I'm affraid i don't know that much about Haltech...