The only problem with heads up is the dealer (and possibly others) can very quickly ascertain your skill level. At a full table it would take them possibly hours to do so if your not being scrutinized (as is the case the vast majority of the time). At an empty table, takes less than half a shoe.
Case in point, a table just got wiped out, and the ploppies were just milling about. I sat down, played half a shoe, and the dealer told the other players they should learn from me
Despite the fact he didn't know BS perfectly (gave me a strange look when I hit A7 v 9).