Exactly what I just wrote in another comment :) Start with simple, "unrealistic" load tests of suspected hot paths and you're likely to get very good ROI on the hours you spend.
the adage from service level perf testing (you don't know what's really slow/brittle until you measure...) holds equally well at the systems level. Yes, you can probably identify decent areas to improve by guessing, but you'll never find your blind spots...