15 / 5 = 3, so that other kill must have been worth 2-3 points, and the multiplier probably kicked in, making it higher still.
I landed a 5 kill sortie in an F6F in FT, not sure of the numbers but we were getting ganged by both other countries. Next sortie I checked "last sortie" in hangar, and it said something like 50 perkies (for ONE sortie!!!!!!!)
Rare, sure, but I've easily made 35 perks in a Ki84 sortie, many times, to tie it back to Gianlupo's example. I guess the perks system works well enough. I'd like to see some changes (i.e. have some perk planes at "1" eny -- or if this is too generous, at LEAST "2" -- "5" is too much like every other free plane in the game.)