"why would anyone want MK23 if you have USP? it just USP upgrade" yes, it is, but it been improve to be better in anyway to be SOCOM grade pistol, here some stuff (thanks to Ian from Forgotten Weapons for this short and simple history)
"The H&K Mk 23 pistol was developed in the 1990s for the US Special Operations Command and US Navy. The goal was to produce an "offensive handgun" that could serve as a primary armament for a special forces operator as well as a backup arm. It was required to be no more than 12 inches long, fit a suppressor and aiming module with laser and illumination options in both visible and IR spectrum, have at least 10-round magazines, chamber .45 ACP (specifically a 185gr +P loading), and pass a 30,000 round endurance test. Only two companies were able to supply acceptable initial pistols; H&K and Colt. The Colt pistol failed to pass the 1st phase testing. H&K presented a gun based on the recently-developed USP design, was ultimately chosen as the project winner; adopted as the Mk 23 pistol in 1996. The testing this pistol went through during development is really quite remarkable."
it also call "Match Grade Pistol" (refers to quality firearm parts and ammunition that are suitable for a competitive match)
can be use with KAC Suppressor made with the gun, and also can use Military Grade gadget as LAM (Laser Aiming Module, which has been create before) to improve capability in combat, mostly in stealth mission
for in-game stats, i might suggest to be x2-x3 of USP stats with mostly improve of accuracy and damage
for me, i have personal Airsoft version of it, with correct KAC Suppressor and Incorrect LAM (it phase 2 LAM design) (for any mod maker willing to take a close look to the gun itself, i can provide you with gladly help by taking photo of each side of the gun, i would be proud to help you ^^ if you willing to make it
more information about the gun itself (also can provide you helping to get correct marking and parts easier) (it have Phase 1, 2 and 3)
https://www.youtube.com/watch?v=1lJeTkgMKqo
if any mod maker like to contact me for a closer look of my MK23, i can texting with you and give you the all-around of the MK23