tank cannons sounds are broken in 2.4 due to the way they are mixed with distant sounds. That is why you think they are "low". They aren't, it's just that they get muffled by the distant overlay.
There ofcourse is limitations on how large you can make a muzzleflash without lag or just looking plain silly. FH1 cant be compared since it's different game, but ofcourse we could make the tank cannons muzzleflashes larger, I can look in to it perhaps, allthough I think the current one lets you see that the tank fired well enough, which is what their job is.