I like the 10mm in both revolver and auto. One nice thing about the moon clipped 10mm's it the ability to shoot 40S&W ammo. On the same note, you can pick up a KKM 10 to 40 conversion barrel and shoot 40 out of the G20. Not sure if they will do a conversion barrel for the G40.
I think I'd look for a S&W 610 rather than convert a GP100. It would probably be about the same cost (via Gunbroker) and no wait for the work. You'd also likely be able to sell the 610 in the future for no loss...which is doubtful with the custom GP100.
...now if you want to convert a 4.2" SP101 to a 5 shot/moon clipped 10mm I'm all for it! ...and I'm willing to purchase it from you for ~1/2 price a year from now!
