I am seeking a highly accurate varmint load for the 444 Marlin with 200 gr. JHP.

Haven't tried anything less than 240gr (which by the way, makes quite the mess of gophers) but on loadswap.com they had listings for 180gr and 200gr loads. Good luck!

The maximum effectiveness of that bullet is about 1600 fps. Anything over that and you will explode it and on big game produce shallow wounds.

I recently tested that bullet ( 200 grain XTP) using SR 4759 and chronographed it from 1600 fps down to about 800 fps. I can attest to its game getting potential based on the backstop damage to the tree behind the chronograph.

I can't attest for accuracy in any one else's 444, but mine seems to be very accurate with no matter what i feed down the barrel. If you are interested in that data, let me know.
Cheers
thunderbolt.