I wanted to investigate about weight savings (and, conversely, weight increases) got removing/adding gunpods or outer wing cannons, exploiting loadout options offered by stock game. Tests performed with game version 4.14.1 and, for comparison in some instances, 4.13.4 and 4.12.2.
I got a couple of oddities. One about MG 151/15 under Bf 109F-4 wings, that does not work with stock game (at least I'm unable to get them), but only works with Anto's mod "Bf 109 Ultimate Pack". Barnesy's "Bf 109 ETO Pack" sets them as MG 151/20 with an additional weight of 393 kg. The other one affects Fw 190A-6, whose outer wing MG 151/20 weighs only 36 kg!
According to what I indirectly believe to have understood, additional weapons should include a "ParasiteWeight" and, where applicable, a "ParasiteDrag".