Diablo type pellets are actually most accurate at about 800fps or even a little slower. The marksmen operating the airguns are more accurate at 850-900fps simply because it trims operator error to an extent.
Back when I was tuning and shooting my .25cal BSA Lonestar, for 100yd groups, 800fps in good conditions is where I saw the best accuracy, actually astoundingly good accuracy. I've noticed the same in .177 and .22cal shooting field target. On the practice line at my first National championship match (Kentucky 2008) I had my AAS400 set up to shoot about 10.3's at 880-890ps. It was shoot decent groups, not stellar, but consistent 1-1.5" groups. It was getting close to refill time, but I was feeling pretty focussed, so I kept shooting. All the sudden i was putting pellet on pellet at 50yds!! Not talking 3-4 shots mind you, but a dozen or more.....Took me "a minute" to put two and two together, but later I shot a chrono string at the lower fill pressure and discovered it was approximately 800fps and less that was punching the same hole like that...not 890fps.