I disagree oh so much I want to shout!
8kg is about 16lbs. Keep in mind a lot of the Hispano armed planes have a lot less ammo than the other armed planes (consider n1k2, fw190s, 109s even! Ki's, C2, etc etc...).
Planes with hispanos are at no weight disadvantage, at all!
Take a spitfire, sit on the runway. Pull up E6B, note the weight. Fire all cannon. Note the weight. Divide the difference by 240 (pounds per round, basically).
All in all the Hispanos are the most lethal cannon in the game for range, aim, and reliability. They weigh almost the same as the average 20mm cannon in this game, and the round itself isn't much heavier than the average 20mm, and it often carries less (read: weighs less) than contemporary 20mms.
IMO Hispanos have no drawbacks.
EDIT:
Weights I already checked (months ago) on the 190a8, ammo only.
950 13mm rounds = 171 lbs
500 20mm rounds = 242 lbs
280 20mm rounds = 136 lbs (outboard)
110 30mm rounds = 143 lbs (outboard)
Sure it's the extreme, but almost 250lbs of inboard 20mm alone! Forget about the n1k2! Forget about a 109 with gondolas! Heck, even a 109 with just the nose gun carries more RPG than the average spit (150 rounds) and the F-4/G-2 have a 200 round option that is almost twice the RPG of the spitfire, all for a single gun.
EDIT 2:
109G-6 (150 rounds hub gun) with gondies:
Basic 20mm ammo weight = 73 lbs
Gondola 20mm ammo weight = 121 lbs
Total gondolas weight = 507 lbs
Empty gondolas = 386 lbs
each gondola = 193 lbs
I doubt spits carry that much weight around, and are probably more lethal than a gondie carrying 109.