.22 HW30 is unquestionably more efficient than a .177 they are no diferent to any other gun bigger less air or in a spring less spring force its that science thing again.
HW30 in .22 get around 9fpe not much more, and a .177 HW30 with the same spring force will be 8 ish fpe.
Weight is where the HW30 wins big time over pretty much anything, and as its such low power the spring force involved at they powers is not significant and the shot cycle and indeed cocking effort are sublime.
The HW99 is heavier its balance and handling not as slick as a HW30, its in the 11sfpe from the factory and given its 7lb umscoped weight although not a massively heavy gun its heavier than the HW30 and to be as slick it will need to be late 9s early 10s in .177 and emid 10s in .22. which will involve detuning the kit adding cost the work or cost of detuning, something you need do none of with the HW30.
If you are going HW99 weight IMO go that bit more get the HW35 and if you are going HW35 you may as well go HW97, the OP is 25/30 yards ... for me the HW30 has it . YOMV.