I would not put anything bigger than a 40 amp breaker on a circuit with 8 gauge wire.

Specific wire sizes mandate breaker size.

Is this circuit a dedicated circuit meaning the Heat Pump is the only thing on the circuit, or do you have other items running off of it like your pool pump etc.?

How long is the actual circuit from the Service Panel to the unit?

If you have a long run, do like Ben said and go with a #6 wire and a 50 amp breaker.