Considering the fact that you have a SRX5600 and no NAT there should not be a lot of performance impact but we really cannot say as much until we know how much SIP traffic you expect the FW to handle.
Regarding configuration, you may want to check the below mentioned documents. The first of which is a note which explains the working of SIP Alg and second is a configuration guide along with probable scenarios.
In my opinion sip and sccp together(I think), improve performance because the common network range is extended to the mobile device sector of networking. Using only sccp will not bring gain. Sip will though. Phone switching will always bring broader performance but round trip could potentially get much longer. You may have to multi-home properly so they both jive. One protocol alone is not as extended.