[bojan]

Unipolar motors CAN be driven by bipolar driver (but bipolar motors can't be driven with unipolar driver).
You just have to use the central tap and one end of coil (if you use the "wrong one" motor will step in other direction).
Connecting the whole winding (leaving central tap "in the air") effectively increases coil inductance, hence reducing torque at higher speeds ("no guts")