Hi Tom. If the Leslie Sim is very good(and only a handfull are) and it is used correctly then from an audiences point of view they will not know the difference between the sim and a miked up leslie. However you have to do it properly.

Almost everyone makes the mistake of using the full stereo image for the Leslie Sim. The Hammonds sim (which, like 99% of sims including the ventilator) is two dimensional - in other words shifts from left to right) it is vital to run the sim through a mixer, take a look at the position of the keys on stage and then 'pan' the left and right channel to that point.

The only exception afaik is the KeyB which uses a 3D image and sounds fine just DI'd straight into the PA or through a Stereo keyboard amp in your living room. However for recording purposes it still might be best to place it 'somewhere' in your stereo image rather than taking up both channels fully.