taken from grandprix.com:
Wind tunnel models incorporate small, ejector-type, air driven pumps that simulate the air flow into the ram intake and augment it as the exhaust flow. The effect of throttle-open and -closed can be tested and the changes in downforce and, more importantly the centre of pressure, can be measured. In recent years, the effect of the exhaust on the centre of pressure, and its variation with throttle opening have led to the true, blown diffuser being abandoned, and exhausts exits moved to blowing over the top of the lower body rear edge. In this position, they probably did more to increase the radiator air exit flow than influence the underbody flow.http://www.grandprix.com/ft/ft00360.html
Article is from 2000 though. I'm sure things have developed significantly since then.
It does seem it is difficult to simulate exhaust given how Red Bull and Ferrari struggled to get it right in the early part of last season. Mclaren did get it right whilst Mercedes and Lotus went with solutions that were already tried and tested pre-2010.