I've pushed my old BSA Scorpion from 720 fp/s to 860 fp/s with 10,34 grain JSB and it stays accurate at any speed. Even with HN Bara ore FTT, RWS. IMO one of the best airgun barrels.
However my Vermi is a great airgun but it needs a good barrel to perform on the 3 power levels. So far I have my doubts about the ST barrel. I owned an Bobcat with ST barrel in .22 too...same problem, once the velocity drops it more ' seeding ' than grouping.
BSA Scorpion .177 17 lbs Humareg, Falcon Menace 4x14 FFP
FX Verminator MK2 .177 19 lbs, Falcon Menace 4x14 FFP