A tune of modest velocity using very short valve dwell can be extremely efficient. The Impact has a 480cc bottle so your efficiency works out to 2.41fpe/ci.
There have been quite a few well-substantiated tunes producing over 2fpe/ci and a couple that approached 2.5 but they required significant modification, custom parts, and tuning effort. One notable example is motorhead's heavily modified Marauder with full-bore porting, a custom valve, lightweight hammer, and SSG (anti hammer bounce device):
https://www.gatewaytoairguns.org/GTA/index.php?topic=148788 That's a kind way of saying I think your numbers have an error or two