What dyno is capable of showing vacuum at idle?
Com'on. Can even any eddy current setup chassis dyno hold steady or do a stepwise down at 1000 never mind 650 rpm?
You want to test a bunch of theoretical changes? There's several decent simulators including pipemax, dynomation, and even more expensive ones. There was a program that used to come (or maybe it was optional) with some of the Speed-talk Horsepower chain packages. Dynomation's F&E model will provide lower rpm (maybe down to 1000 or 1500 rpm) but I wouldn't rely on it being that accurate.