Firewall HA, Active -Active Setup

Personally i would not recommend A-A, instead take a bigger appliance with A-P. 
Reasoning: The license module of SFOS needs two subscriptions for A-A and one License for A-P. So to speak, you will likely be having a better offer for a A-P appliance of a bigger appliance. 

A-A does not offer 100% more performance as it is not a full Load Balancing, instead certain tasks are being load balanced. A-P has the advantages of having still the full performance, even if one appliance is faulty. 

Thanks Toni!