With the exception of the specialty ammo they can use. That's what keeps them viable in the face of LB-Xs and Ultras already.
The specialty ammo is a bandage, not really enough to let them compare to the LB-X and Ultra, let alone these.
For the weight of an AC/2 you can get an UltraLight Gauss with an absurd range bracket increase and a 50% increase in damage. Also, if it explodes it's not nearly as devastating as an Autocannon ammo explosion. That's an upgrade on nearly all platforms regardless of how much Precision Ammo you stuff on it, except maybe something like a Jagermech for the AA role (flak ammo), but then again the #1 rule of AA guns is just "Range" on repeat.
I'd honestly replace any IS AC/2 or UAC/2 with one of those without a second thought. I'd probably replace the IS LB 2-X on most things too unless I really wanted an AA pop-gun. And it compares extremely well to the Clan LB 2-X and UAC/2.
Imagine a Warrior H-7 variant with one. You'd stop the Clan Invasion cold at the Rasalhague Dominion's Borders.
The Very-Light Gauss Rifle (Why not XL?) has a similar thing. You pay an extra ton to upgrade AC/5s to a VLGR and you get a rather crazy 50% range boosts, and a more mild explosion compared to an autocannon hit. Considering the state of the AC/5 that seems like an absolute win to me.
Drop a ton of SRM ammo from the SHD-2D and replace the AC/5 with a VLGR, and the 'Mech all of the sudden is a respectable sniper.