I picked up my first Witness pistol about 2 years ago. It was a used steel frame in 9mm. This not one of the new "Full Size" frame pistols. It is however my first DA/SA pistol and I LIKE it alot.
I have sinced purchased a New Full Size Frame 10mm Witness. I paid $400.00 OTD and was given a box of Winchester 10mm 180gr FMJ ammo. This is one of the best pistols I own. As for power I think I was told by a friend who knows that the 200gr 10mm JHP bullet will closely match veocity with the 200gr 41 Magnum round. So IMO I consider this to be a 41 magnum semi-auto pistol.
Right now I'm using the last of a batch of Winchester 10mm Subsonic 180gr JHP LEO only bullets. Besides that I use 180gr or 165gr FMJ's. I'm saving the 200gr JHP's for something special.
As for the Witness series of pistols once you purchase a "Full Size Frame" pistol you can switch calibers by changing the slides and magazines
. Now in the case of the 10mm/40cal. switch all I did was purchase a 40cal BBL and used the 10mm mags with no problems. However I did buy some 40cal mags anyway. I plan to purchase a 45acp set up and maybe a 22cal top end.
All in all I feel this pistol os quite a good deal for the money. I see them all the time on GB.com amd GA.com for under $550.00. G-Code makes poymer rigs for this 10mm and others and Haugen Handgun Leather made a Shoulder rig for my 10mm for less than a Galco or Bianci cost.
So IMO go with the 10mm or 45acp to start and then add the other calibers as you see fit.