So the Ammo backpack (x5 ammo of weapon) is +15kg, which seems simple, except for the Hotshot weapons

Hotshot Laspistol is 4kg
Hotshot Lasgun is 6kg

But the ammo pack for them is 10kg

So the question is, is that 4+10+15/6+10+15 or 4+15/6+15?

I tend to lean toward the former, but the latter seems a bit too light and makes this upgrade even more mandatory than it already is.

Hotshot weapons already use a backpack by default. Common sense says you can't carry two backpacks.

When the rules say that hotshot weapons can be linked to larger backpacks for more shots, I'd say it is an "or" kind of thing rather than "and". Meaning, yes, it IS only +15 rather than +10+15, but at the same time you don't gain the ammo cap from the standard backpack.

Unfortunately, the rules are kind of silly and unclear here, but personally I'd go with 60 shots for the 15 kg power source, because the standard 10 kg give you 40 shots.

tl;dr:

RAW : it's gun weight + 15 for 200 shots (+5 kg for +160 shots, aka +33% weight for +400% ammo)

Common Sense : it's gun weight + 15 for 60 shots (+5 kg for +20 shots, aka +33% weight for +33% ammo)

Your call.

Sorry, I didn't mean getting the ammo capacity, but rather just the weight, if the ammo weight is +15kg or +25kg (either way, 5x ammo)

Well, the answer is still the same: it depends on how closely you wish to stick to the rules over what would make more sense.

I may have to correct my previous summary, though, as I suppose "can also be linked to larger backpacks" should be read as an "instead" rather than "and". This at least means you wouldn't carry two backpacks after all, so it's +15 kg, for a total weight of 19 or 21 kilos.

If you really want to stick with the RAW and use large backpacks for hotshot weapons as they work for any other gun, then yes, they do sound pretty mandatory. And their efficiency is even more ludicrous, as now you've got a 150% weight backpack giving you 500% the ammunition. In my game, I'd argue against the ammo capacity, as it's straining my suspension of disbelief, but again: your call.

To be more precise, this strangely high efficiency in terms of weight vs yield is, as you said, unique to hotshot weapons, because these are the only weapons in the core rulebook that already come with a (smaller) backpack by default, and are explicitly allowed to exchange it for the larger version. It would have been less silly in DH1, where the hotshot weapons in the Inquisitor's Handbook could also use standard chargepacks, but consume 4 shots with each pull of the trigger, which would have allowed to extrapolate a somewhat more sensible ratio here (= standard chargepack / 4 x 5).