Mmmkay now. Here's some numbers for comparison to things which are not Kasrkins. Pure numbercraft, but still valid.
Important Basic Facts: Kasrkin Hellgun range is 40. This is higher than anything else in the game that's not on a vehicle, Tau, or not an IG Grenade Launcher.
Flash Gitz and Heavy Bolters come close at 35 range, and Tau Fire Warriors are also at 40 range post-Improved Power Source, 50 range after Kauyon range improvement. Tau Bodyguards do 70 range.
The Frag Grenade does ~100-175 damage to infantry in a radius of 6.5, which is inbetween a Basilisk round and an Earthshaker Round. Recharges in 30 seconds.
A critical component of Kasrkin usage involves Execute.
Execute may or may not be bugged at the moment - there's talk that it may override the Priest bonus or that it may even affect the whole squad's weapons, not just the Hellguns. These figures use the intended idea of just buffing Hellguns and Priests stacking on top of that.
Basic Kasrkin damage is rather underwhelming. It's 12 DPS to heavy_high, 16/17 to heavy_med and infantry_high, and 19 to infantry_med.
With Execute, though, those DPS values double plus change -- 25 to heavy_high and 33 to heavy_med. This is approaching competitive levels.
Add a Priest and fun times begin - 50 DPS to heavy_med and ~38 to heavy_high.
Kasrkin Plasma Guns do meh damage base. When Specialization and Priests are added, they hit 60 DPS to heavy_med and 48 DPS to heavy_high apiece.
Sergeant does about 50% more base damage with his Hellgun. 33 to heavy_med and 26 to heavy_high with Priest bonus.
So, let's add things up.
9 Kasrkins alone, unbuffed -- 108 DPS to heavy_high, 144 to heavy_med.
9 Kasrkins with Execute -- 225 DPS to heavy_high, 307-ish to heavy_med
9 Kasrkins with Execute / Priest (assuming it works as intended) -- ~347 DPS to heavy_high, ~450 to heavy_med.
9 Kasrkins with Sergeant, Priest, Execute, 4 have Plasma Guns with Specialization -- 415-odd DPS to heavy_high, 522-odd DPS to heavy_med.
That's a lot of effort for some decently impressive damage. In theory.
1 squad Flash Gitz - 320 DPS to heavy_high, 360 DPS to heavy_med, range 35
1 squad super-buffed Warp Spiders + Exarch - ~285 DPS to heavy_high, ~342 DPS to heavy_med, range 25
SM Tactical HB squads do 250 DPS from the HBs.
CSM HB Squads do 224 DPS from their HBs, 280-320 DPS with Berzerk Fury.
CSM Plasma Squads do 270-odd DPS from Plasma, nearly 390 DPS with Berzerk Fury to heavy_med.
So, with one Kasrkin squad, lots of upgrades, and a Priest and Executing Commisar, you'll get about 1.5x more firepower than a single squad of ranged specialists in Tier 3 and a lot more range.
Kasrkin damage is really variable though - they've got 50% accuracy and fire once per second, and with Execute their maximum damage is through the roof, so a few lucky shots will really make a big difference. Unlike other ranged troops whose DPS is pretty close to actual damage per hit, Kasrkin average damage per hit is basically taking their DPS and multiplying by x2, then another 50% or so to see the max damage per shot.
Combined with the Frag Grenade O' Doom making a nice head start, that is another factor.
But, yeah, 9 unupgraded Kasrkins just with Execute match a HB squad in terms of sheer expected firepower.
Problem is, there's a hell of a lot more HB squads, Warp Spiders, and Flash Gitz than Kasrkins.
But, still, there's some comparison numbers for you. With that kind of carnage, just imagine the fun to be had from just two Kasrkin squads alone -- twice the Frags O' Doom, twice the Hellguns, only one Execute to turn them into gods.
Figured I'd throw that out to whomever's interested.