Do you have your bike lowered in any way to accomodate your shorter stature? Do you have any handlebar modifications to put the bars closer to the rider?
I have a friend of 6'4" who bought a Sprint RS and from the seat height specs those bikes sit just 5mm higher than a stock ST. He was comfortable on the RS, though what the specs don't show is that those have the bars 25mm lower and 25mm further forward than an ST.
Have the potential customers go to this website, enter in the riders height, inseam and even arm length and the website will show his theoretical frame seated on the exact bike you select. In this case of course it would be a stock
1999-2004 Sprint ST. Note that there are optional boxes to open up to place the rider at the front, center or rear of the seat.