So here's my quick tip for selecting a WatchGuard M Series firewall to meet your needs. Having sold and managed literally thousands of firewalls across the world since I started in 1999, I have noticed one thing - Our browsing and data consuming habits effect a firewall's throughput. All the tests above were preformed in labs and under controlled conditions.
I would always recommend taking the slowest speed published - in this instance something like the HTTPS performance value and then dividing it by three. This will give you, from our experience, the worst speed you will get under the heaviest load conditions. Make sure this speed is faster than your internet connection and you will be good to go.
Example using a WatchGuard M290
HTTPS speed is 696 Mbps, divide this by 3 which gives you 232 Mbps. This is the slowest we believe this firewall will run under the heaviest of loads. Obviously if you want to turn some of the security services off, then you will make it run faster.
Don't take my word for it though. Have a look at the real world performance tests we have conducted below.