I cannot recommend more than 1.2 amps.
The chip itself is rated 3 amps maximum, but in my experience it's tempting fate to use more than half of the maximum current.
The heat sink is running at maximum with 1.2 amps. Any increase would need a larger heat sink.
Incidentally, power transformers vary in their open circuit voltage and DC resistances. The board works with the Kaiju power transformer, but other transformers with the same spec may or may not be able to offer enough voltage headroom.
In other words, it was never designed as a general-purpose filament power supply.