A lot of the evidence around re-designing the shiva variants is going to depend on pilot (and victim) experience.
My experience with the various shivas (as of recently) -
Shiva Prime - Rarely (if ever) used. Nothing against the prime loadout, as I've mentioned in another thread regarding variant rebalancing. It's more a matter of the current state of the game, and other players' habits, results in me having to choose a much more specialized bird.
Shiva A - I should really have looked first at which variant has what. Is this the UAC loadout? To hell with it, I'm assuming this is the UAC loadout. This is one hell of a flying short-barrel revolver. Big boolet, big damage, crap range. This makes it incredibly fun if there's a weakness in their AA that allows you to get close enough to hug mechs with the UACs. In any other situation, it's pretty much a suicide run.
Shiva B - This is the ELRM build right? As some have pointed out, this is pretty much a one stop shop for uber-long range harassment. Short of having ARROW IVs on it, this is about as deadly a missle boat as the game currently has. I would probably suggest dropping the NARC launcher for some sensor tech, GECM/BHP/AECM, to give it a bit more survivability at the cost of making it rely on a dedicated NARC/TAG or really bad Anti-Air from the enemy. Lock on times would make this very prohibitive without a NARC/TAG, but that would balance how powerful it's salvos are.
Shiva C - The PPC + FBOMBs eh? Honestly I haven't got to spend as much time in this bird as I'd like to, given the Khan++ cost. If I were to propose any changes to this already very powerful build, it would be to replace the PPCs with something slightly less potent, making it merely an "emergency" pair of weapons. Maybe a bunch of ER MBLs? Or a pair of LPLs? Aero generally don't work well with beams, given their twitchy nature, so maybe going to something like a pair of Light Gauss would work too. That way the Shiva isn't helpless without it's Fbombs, but it isn't still a threat to anything other than light anti-air pursuers.
Shiva D - I'm going to assume this is the Heavy Gauss Shiva. the machine guns are 99.9% useless on such a bird, so I agree with some of the already proposed changes. Either give it a faster firing anti-air deterrent that actually does some damage (like a LBX5 or something) or just give the bird more ammo space for the HGauss. I haven't flown this enough to really give it a fair shake, but having taken a pair of HGauss hits from a guy pursuing me with such a bird once, I can say this spec is already very deadly at the anti-air game.
Shiva E - I only recently felt the time was right to give this one a try. Honestly my experience was really a mixed bag. Against any ground asset lighter than a hollander, this mech is a joy and a half. Swoop down, mince up ground targets, then run like mad from the hornets you've stirred up. Against air assets, this is not quite as deadly as it would seem. The RAC5 doesn't do a whole lot of damage, especially during the brief chances you have to hit during aerial combat. This combined with the spin-up time, 300 round ammo pods, Lag, and differences in aerial agility make this really only possible against other Shivas, or really REALLY bad pilots. VTOLs are an exception to this, as they are fairly easy to line up and unload on. If I was to make a suggestion, it would be to drop the MGuns and maybe a few tons of ammo for another RAC5 installment. That or supplement the RAC5 with a comparable fast firing weapon with slightly less damage that has instant firing capabilities. LBX5 comes to mind, but I'm sure others can come up with some good suggestions.
As a mech pilot, the only truly infuriating moments I've had against air units are the sudden Tbolt/Fbomb hits, and the NARC + Missle spamfest. TBH this will happen even if the ELRM shiva gets a heavy handed nerf. The cost of specialization should be a critical weakness, and really all the ELRM shiva needs is to not be a one-stop shop for missle spam fun. This is already well balanced by the fact that none of the big missle boat mechs have NARC launchers. Thus relying on aero or scout mech pilots to mark targets. The same should apply to the ELRM shiva, and even possibly the Tbolt sulla. Make it so that the NARC launching sparrows have a clear-cut purpose on the field, and we'll definitely see a better balance of power from the air.