Resonable Stepper Speed under simple Unipolar drive ?

Hi all

What is resonable expectation for stepper speed when running
a Unipolar drive ?

Specs:
12V supply voltage
4.3V @3.5 Amp motors
Ballist resistors to control current
Unipolar mode

M93 series Slosync motors
400 oz/in


Currently under Mach1 I can achive 225-250 full steps/secs unloaded before it
resonates and slips phases.
Under load I can get around 300 full steps/secs before it resonates.


Does this sound typical or do I have problems ?

Thanks
Mark