check out the tomahawk IPS test tool - http://www.tomahawktesttool.org/ Throwing one attack at a time through the IPS is going to let you know if it has the capability to block that specific attack, what you also want though is to know that your IPS can block multiple attacks coming at the same time, or 1000 instances of the same attack etc.. and metasploit will not do this on its own.

Using tomahawk you replay TCPdump captures of attacks and see what happens. So perhaps launch an attack using metasploit, capture it with tcpdump, then replay, and then repeat.