Until now, in order to verify your firewall supports the numbers described in its data sheet (connections per second, latency and more), you needed to invest lots of $$$ in expensive proprietary testing gear. 
This is not the case any more!!! 
No matter if you are a customer, QA engineer or developer, you can now easily do all these tests using the TRex realistic traffic generator.
At a fraction of the hardware cost (running on Cisco UCS), and with the software free, you can now test full 40G (100G under development, will be available soon) line rate of TCP connections.
TRex can simulate hundreds of thousands connections per second, and it also supports NAT and TCP sequence number randomization in client->server and server->client directions.
TRex is already heavily used by many groups within Cisco and outside of the company (Intel, Mellanox and more).
And, the best thing is that it is developed in house at Cisco, so new features can easily be added upon request. 
Check out the TRex Documentation to see what else it can do for you.
For ASA testing details look here:
https://trex-tgn.cisco.com/trex/doc/trex_manual.html#_trex_with_asa_558
