As I can rmember, the vector will be on 45 degrees with the same force in x and y direction, so I think it is about 1.4 times the torque.
Consider the inductance of the winding, if you drive the windings with a constant voltage, the current will rise according the inductance of the windings (V = LdI/dt). At higher stepping frequences, you do not reach the nominal current anymore, and you loose a lot of torque. When you put the two unipolar windings in series, you get a higher inducance. You can use a chopper circuit to have enough current at higher steprates (sinde the initial voltage is much higher, the slope of the current will also).
Stefaan