Try to increase your supply voltage to 36 volts (about the practical maximum for L298). More voltage means more maximum speed to your stepper.
Another thing to consider is that a stepper alone can't usually rotate very fast, as it needs something as a load to damp the resonant vibrations. A little friction on the shaft or mechanism won't do any harm - on the contrary.
And of course, the acceleration must be related to the voltage used and the inertia of the system. And remember to use a very even pulse stream, as an uneven stream of pulses is seen as a quickly varying speed command by the stepper, which it may or may not be able to obey.
I think that you'd better use two L6203 drivers instead of one L298. The former is a FET based driver that runs cool and can deliver more current than the latter. Usually the reliable maximum current for L298 is about 1.5 amps, with more current it usually fries to death, regardless of the heatsink. Besides, it's slowly becoming an obsolete component.
BTW: I think your motor's rated torque is 2.7 Ncm, not 2.7 Nm...
Happy stepping,
Hobbie