Well, my report is pretty pitiful... I dicovered:
1] my shop is a disorganized mess, with 748 and 750ss parts all jumbled together, and I need to do something about it.
2] I was wrong about the Barnett spring cups, they are the same depth as stock. the difference in spring length I mentioned is due to the fact that they are not for a dry clutch, they are 40mm free length [which is the proper length for wet clutch ducs.]
3]I did though, measure a spring cup that BCM sent me for racing applications [for a wet clutch] and it is much deeper [fully 6mm deeper]. Of course, I could not find the stock wet clutch cup [see #1], so I have no comparative measurement for you.

Sorry, I wish I had more input. Ultimately, I'm with all the other guys. Try it without 2 springs...if it works for your riding style [as in; how much time do you spend with the throttle whacked wide open?] you win.
I do like DesmoBob's idea of shimming between the cup and post. It sounds like an easy way to tune the tension, and guarantee uniform pressure.