From what I have seen, the performance package (at least on 2010-12 models) doesn't help that much, as long as you lower the car with H&R springs... I cannot compare PP vs non-PP for the newer model, but I had a non-PP 2010 and now a PP 2013 and it really launches faster now. However, the tuning is not as aggressive yet on the 2013, so I launch faster, but I lack of top speed.
My non-PP has run many runs from 12.0-12.3 on different tracks, so the PP on first EB SHO generation can be matched with a non-PP with H&R springs.
One thing also, for a 0-60, the non-PP, once tuned, can reach passed 60 mph in second gear... The PP needs an extra shift, so it is not faster from 0-60. Also, I would think that the top speed could be higher with the non-PP...
In your case, I understood that you were looking for a used one, probably 2010-2012, so I would say go with the best deal...