I think 7 pen on hell weapons if fine. Given their damage is static unlike a power weapon which can get considerable strength bonuses going for it. Both are made for fighting power armor or heavy infantry. Hot Shot packs allow snipers or other specialists a one shot anti heavy infantry weapon.

The mechanic is that the hellgun AP is high but the damage only ok. While plasma does huge damage and a point more AP and power weapons use muscle and a good base damage to cut through armor not just a high AP.

A hellgun punches a neat little hole in its target. It can wound a normal man in a sweater just as easily as a carapace armored grenadier but that high AP punch is the only thing that allows it to potentially wound a spacemarine.

I have always ruled that unless you are running a couple shots per laspack and reloading all the time you have to wear the backpack unit which means you can only wear the combat vest that carries 15kg of extra equipment rather then a full backpack full of extra ammo and supplies.

As far as the tabletop rules and fluff influencing the pen and paper. the strength 3 AP 3 hot shot/hellgun in the guard codex can take on marines somewhat effectively. In DH however if the gun was only Pen 4 rather then 7 the max damage is far too low to fight the armor value that its meant for. As well unlike the tabletop a normal lasgun doing 13 damage max cannot wound a space marine except on righteous fury.

This entire thing is just the cumulative effect of canon dis-continuity. 40k is a competitive wargame which has lost a great deal of its internal balance thanks to the majority of players having armies build of soldiers with T4 and 3+ saves. As a result the codices (its not codexes, **** off) in the 5th edition of the game made a lot of changes based around this 'meta' aspect of the game rather than creating a fully balanced system between armies (something they are now struggling to adjust, because the power creep was reaching critical mass where no level of armour was worth anything anymore, I still see people complain when any gun is less than STR 10 AP1, literally as powerful as the rules allow it to get). One of the ways they tried to adjust for this was to introduce more AP3 and below weapons to the game, and the IG hellgun was one of these that got the boost, contrary to previous lore.

FFG have taken the latest (current?) incarnation of this lore and made a weapon that reflects its rules in the table top games current ruling. I really fail to see why this has generated 3 pages worth of arguments (lets be honest, AP6 for power weapons vs AP7 on a hotshot gun is pretty moot). Currently what they have established IS accurate to the game, but the game itself is not consistent with its own established lore.

You should read this article by Aaron Demski-Bowden on the subject. It pretty much sums up why this is not a resolvable problem for fluff purists (which btw, is an oxymoron because the fluff is always changing)

http://www.boomtron.com/2011/03/grimdark-ii-loose-canon/

Also, CPS, FFG acquired the rights to Dark Heresy in 2008 which means we are about to hit the 6 year mark for the time they have been developing the series. Just going past half a decade of development is not 'nearly a decade' by any stretch.