The exit hole is essentially just the last baffle.
Reducing the size to suit .177 would certainly improve the performance...

The trouble is that for sub 12ft/lb air rifles there is so little air exiting the barrel that the different is somewhat negligible.
That's the main reason that most companies only off a one-size-fit-all option for air rifle sound moderators

Redesigning the shape of the baffles and air chambers can also up the performance, but there's always a cost; price, weight, accuracy etc.
It's usually a good idea to remember that the popular moderators on the market were designed by a bunch of skilled engineers and optimised for maximum results on a given budget.