There is no problem using a Variac with fixed taps for up to 1:2 ratio step up, as long as you never exceed the rated wiper current (other than as specified for short term overload).
Also, to avoid saturation, the input may not be applied to a winding tap with a lower rated voltage than the supply voltage.
At ratios above 1:2 you have to derate the output current so the current in the lower part of the winding (between the L in tap and N) does not exceed the rated wiper current:
From KCL, Ilower=Iin-Iout.
Iin*Vin=Iout*Vout
Iin=Iout*Vout/Vin
Ilower=Iout*(Vout/Vin-1)
=> for Vout/Vin>2, the derating factor is 1/(Vout/Vin -1)
For a Variac like the O.P's one, that outputs up to 270V and 2A at 230V input, with 115V input applied to a suitable tap, so that the maximum output is still 270V, the derating factor is 74%, so the maximum output current will be fractionally under 1.5A at maximum voltage. At 250V it will be able to supply 1.7A. When set to below 230V, the full rating will be available, but if you want it to be foolproof, use a 1.5A breaker on its output.
N.B. In a 1:2 or greater stepup configuration, the wire to the input tap will have to carry up to double the rated wiper current. If you've got a large Variac, make sure it has enough Ampacity to do so