We have a fun story with a local large bank (largest in my country). They bought an expensive testing tool that couldn't load jPOS beyond 455 TPS. That was 10 times larger than the load the customer wanted, so they were happy, but we weren't, so we used the jPOS client simulator, and drove that very same system to 4400 TPS. Image from the report is unfortunately in Spanish.
So despite my bias of course, if you ask me what's the best tool to test these kinds of systems, I have to suggest jPOS.
I understand there's some third party SOAPUI ISO8583 that uses jPOS and I believe works pretty well. Didn't use it myself.