My advice is to not try for the "hail mary" end-to-end, soup to nuts workflow. Take each integration point as a separate entity, and push volumes through these points according to the NFRs. You have to insist on a level of granularity in the NFRs that will allow you to do this.
For those endpoints that are monitorable, use SiteScope to collect host performance metrics. For those that are not (DataPower), just concentrate on transaction times. I don't know what you are referring to when you say "monitor the WSDL".
Good luck - GuideWire is a relatively stable solution, but the data conversion and systems integration pieces can be a nightmare to try to performance test around.