Pump flow rates are determined by the amount of head loss in your plumbing. For a Whisperflow, a TDH of 60 will give you over 110 GPM for a 2 HP. If you only ned 56 GPM, then you only need a 1/2 HP or 3/4 HP. Here are the pump head curves:

http://www.poolplaza.com/WhisperFlo-techspecs.shtml

The lower the HP, the lower your TDH and GPM will be.

However, since you have a spa, you may need the higher HP for the jets, assuming the pump runs both. If so, then I would go with a 2 HP 2 speed pump to get the high HP for the jets and the low speed for general circulation.

Another option, if you have extra cash to burn, is the Intelliflow 4x160. This will give you 4 settable speeds to customize the exact flow rates you need.