Visually speaking, why does the Heavy Plasma has a scope if it has such poor accuracy? Also, I'd spice up its auto-fire accuracy a little. It's might weak now. You seriously nerfed one of the best weapons in the game to a nigh-useless status. Remember that in higher difficulties, the heavy plasma is one of the few guns that can successfully pierce sectopod armor. So either make the Heavy Laser a little more accurate, or keep the Heavy Plasma damage the same with a little better accuracy or maybe no auto-fire mode, like TFTD did with sonic weapons.
By default, the Heavy Plasma's accuracy is second only to the Plasma Rifle's except in aimed shots, in which it's the clear best. For the Terran variants, I only broadened that accuracy gap a bit by making the Plasma Rifle a tad (+10%) more accurate. The Heavy Plasma is unchanged on the precision front, and remains the second-most accurate weapon in the game. Aimed shots, which are the ones that benefit the most from a scope, still have the best accuracy, only now matched by the Plasma Rifle's.
And while it's 0-20 points weaker, it's faster to use and in optimal conditions provides the opportunity to fire a devastating 9-round burst (3 auto shots) in a single turn. It isn't as overpowered as it sounds, and it's a good option to have in some cases. Not to mention allowing troopers to be a bit more mobile and still be able to get shots off.
Going back to the damage front, I'd assume the inability to destroy inner UFO walls is a larger disadvantage than having to fire more shots to kill a Sectopod, an alien unit particularly resistant to plasma weaponry: by default, the Laser Rifle is virtually just as powerful as a Heavy Plasma against Sectopods (90 vs. 92, respectively, which is 90 vs. 84 with this mod). Heavy Lasers are still the best option against them (120)*.
*While Heavy Lasers are otherwise rather underpowered in the vanilla game, they're not made more so by this mod. And modding them is outside the scope of this project.