The sum of all the crypto, in and out, with a reasonable packet size mix can get to 300Mb/s. So if users mostly pulled 200Mb/s and pushed 100Mb/s, that would take you to your 300Mb/s limit.
Firepower throughput varies depending on weather it is dealing with an attack or not, and the type of traffic flowing through it. You might get 600Mbs to 1Gb/s.
Some flows, like a large file copy, will go reasonably fast. On the other hand, something generating lots of small http requests will use a lot more resources and be slower.
Here is another more up to date table fronm the model comparison tool. I have also added the model below (5516) and the model above (5545).
https://apps.cisco.com/ccw/cpc/guest/compare/ucsComparePage?selectedValues=model_asa5516x,model_asa5525x,model_asa5545x,&productFlags=N&callbackComponents=series_asa5500
If you are running into throughput issues sometimes the cheapest approach is to offload all the VPNs onto a dedicated firewall. A 5516 would probably be a good size for you, and not too expensive.
