TCSM are meant to counter infantry. More so on HI or SHI. I dont even think MoT is really need until t3. CSM do fine already with Eternal War in t2. But one reason why I think like this is probably because I usually get 2 CSM when I play chaos.
As I said a long time ago on this thread Tactical Marines are fine as they are. Just the decrease in repurchase of a weapon upgrade would be a large buff in of its self.
Plasma guns are not bad in the least. They do decent damage to HI and SHI. Just like Missile Launcher to a vehicle, decent damage. IMO they are just there to HELP counter and put pressure on a unit. They aren't supposed to be a hard counter like lascannon to vehicles. If they were, the price would be alot higher than it already is.
SG, as was said before are a soft counter to everything. You pay 30 power and what? 100 req to be able to put pressure on anything you opponent throws at you. After purchasing SG what does it cost to get the counter? Pressing a hotkey or click the ammo type.
If you increase the vengeance rounds to this then Kraken would never be used. So if I had the change it i would be only by 2 or 3 instead of 8. Why? Each ammo type should have a draw back. Here is my thoughts on their ammo types.
Hell fire, is not much use on anything other than commanders and infantry. An very effective but situational ammo type. As everyone knows. Well balance ammo type. and true to the lore which is a bonus.
Dragon Fire, counter to cover\garrisons. Although this ammo type seems to be missing something (maybe a damage increase, or damages all occupants in a garrison rather than one), but being able to have a long range unit that ignores cover bonuses is an amazing perk. But with an awesome perk comes a down side. Not much use out side of that field.
Kraken, Even since the buff to this ammo type i felt against changing it since it cant do alot at a long range now. Does decent damage to HI and SHI. If left unchecked for a few seconds they can lower CSM hp by 1\5th. The CSM have to be in the open but never the less, it now makes SG more threat to HI and SHI. The down side of this ammo type should be obvious. Vehicles and infantry (in which SG already has a counter for).
Vengance, the ammo type puts out quite a bit of DPS make no mistake. The ammo type pressures everything, Infantry, HI,SHI and Vehicles. Just cover is an exception (but SG has a counter for this as well). An ammo type that puts pressure on everything should have some draw back. And this ammo type con is in the form of weapon range and reload time. Which makes sense. Even in TT SG has a shorter range, 18 inches when vengeance rounds are loaded in (AP 3 wooooooo, justs Get Hot!

). An effective ammo type should have its draw back. The only buff the vengeance rounds should get is a plus 2 or 3 to range. Negative on being full range, that would be too much of a buff.
What I posted above is what everyone already knows. Before considering a buff we need to assess what the squad already does. It's what I do before thinking about buffs or changes to the unit. And right now I think the SG is one of the most well rounded squads out there, since they can soft counter every armor type, ignore cover and has one of my favorite ability, They Shall Know No Fear!. Changing SG does not seem at all that necessary or needed atm imo.