I think you can get John Holliger to build you that upper with a flash hider if you want the look. That upper is basically equivalent to what the service teams are shooting (the AMU and Marines). If you want to shoot Highpower, you want a good barrel, a float tube and 1/2x1/2 min or 1/4x1/4 sights. The teams use 1/4x1/4 min sights I believe.
As for accuracy, these rifles are good for about 3/4-MOA with many loads, so 3-4" at 200 isn't even close to unreasonable IMO.
FWIW, my rifle is a mostly stock Armalite National Match model with a WOA-chambered 1-7" Wilson barrel. It shoots great and requires almost no maintenance to keep it hammering. Just clean and shoot.
NRA Master, Highpower Rifle, Across-the-Course
NRA Expert, Highpower Rifle, Mid-Range Prone