Relic should really have called every single weapon a different name to stop all the arguments
They kept the same names for fluff purposes, because seriously who would play a WH40k game if there wasn't anything from WH40k in it?? But had to balance the weapons in regards to the army as a whole, and in relation to other armies.
If you have a look the same weapons within the same army, some of them even have different stats. Brightlances on the wraithlord vs the LP's for example within eldar. Because it would be too imbalanced otherwise. (though arguably still is on the WL)
IG plasma wasn't most likely wasn't made more powerful because of an oversight, or some personal insult to Marine players, but because they lack any other infantry weapons able to damage heavier armour and vehicles.
Grenade launchers become relatively useless after tier 1 as most higher tier units are reistant to their knockback.
At present IG has 1 and only one infantry choice for tier 1 and 2 and a little bit into 3 when we have to research for our HUGE 2 other choices. (we do have commander choices but that's not entire units and no IG commander is a gamebreaker alone)
Most armies have a more options than that in tier 1 and only get more.
We also have the massive range of unit equipment options of ... 2 special weapons... GL and Plasma...
On top of that IG squads are still 2 cap minimum, which means they will have in theory the same number or less (in eldars case with an avatar) squads, and potentially even same or less individual troops at each tier.
a) IG plasma was nerfed to marine levels: We would have no feasible way of dealing with vehicles or heavy infantry with our infantry. Guards would become redundant the moment any tier 2 infantry or a vehicle popped out.
b) If the move penalty is in fact a bug and will be removed. IG will find it very difficult to deal with Assault troops, as we then have no way of fighting them in cc, and we can't out dance them because we do no damage.
c) IG Guards were nerfed to their true FLuff standing and kept at the same popcap it would take your entire IG infantry cap to take out one Tac squad of marines. It's just not feasible unless we all play on supercomputers.
(not saying they aren't too good right now and need a slight nerf, but you can't remove them totally)
If in the new expack, IG had a melee unit (roughriders anyone) and Heavy weapons squads, (mm mortars, lascannons, MLaunchers and HBolters) then I would see no risk at all in adding a move penalty and reducing plasma damage.
I know it makes no sense if you follow the fluff. But the fluff simply doesn't apply to a game where you need to balance armies.
Fluff wise one Marine is the equal of 10 Guards at least, but can you imagine the lag if IG had a 200 pop cap and true IG strength units?
Ignore the fluff, and try to balance them without it. Try to balance them as 'anti armour weapon' and 'anti heavy inf weapon' instead of as 'plasma gun' and 'plasma gun' and cry cause the stats don't match exactly.