The Tachyon Lance isn't the main issue; specifically, it's that the Tachyon Lance has a huge range, and range is the most powerful stat a weapon can have since it allows a ship to deal damage without receiving damage. However, beyond some critical point, the marginal benefit of additional range is negligible. It's useful, but not the game changer that not receiving return-fire is.
That's all to preface this suggestion- I think HEF should provide a 50-100% increase to range instead of damage. That would make it MUCH more useful for weapons like the plasma cannon and less so for the Tachyon Lance. I'd even be willing to guess that the Tachyon Lance's range could be reduced to 3000 without it affecting how good it is. A more reasonable nerf would be to increase its fire rate by, say, 10-30%. It's the ridiculously long range combined with its powerful alpha strike that make it such a devastating weapon.
EDIT: I feel I should provide an explanation of alpha strike and the theory behind it for those who aren't familiar with EVE, which I borrowed the term from.
So, let's say you have a ship with 100hp, and let's say I have a choice of 2 weapons I can use to shoot at your ship, but they both deal 10 dps. One option is an artillery cannon that shoots once every 10 secs, the other is a pulse laser that fires once every second.
Pulse Laser:
The pulse laser deals 10 damage per shot, since it has 10 dps and shoots once per second. After 10 shots, you will die. Assuming my first shot is fired at 0 seconds, my last shot will fire at 9 seconds. You will die in 9 seconds if I use a Pulse Laser.
Artillery cannon:
The artillery deals 100 damage per shot, since it has 10 dps and shoots once every 10 seconds. After just one shot, you will die. I kill you 9 seconds faster with artillery than I would if I had used a Pulse Laser.
Now, let's say you have 110hp and you have a buddy with you that has 10hp. In this situation, your ship will die at the 10 second mark regardless of which weapon I choose, but the pulse laser has the advantage since it will then destroy your buddy at 11secs. The artillery has to wait a full 9 extra seconds to kill your buddy, which could be all he needs to get away or kill me.
The dps is the same, but more of the damage is front-loaded by the weapon that fires more slowly. That front-loaded damage is called the alpha strike. It can be very useful, but situational.
Now back to Starfarer, the game mechanics heavily favor weapons with a high alpha strike, mainly due to the way armor works; more of the damage is applied to the armor rather than absorbed by it when the per-shot damage is higher. When combined with the previously mentioned benefits of alpha strike and the extreme range which allows you to shoot without fear of receiving damage yourself, you have yourself an extremely powerful weapon.
That in itself wouldn't be a problem, but there's another problem that exacerbates the situation. With one exception, the only ships that can mount the Tachyon Lance have incredibly strong tanks. There's a reason that fleet ships in EVE that have extreme range are glass cannons- they sacrifice the ability to absorb and deal damage to outrange their opponent. One of the ways for you to counter them is to get close enough to shoot back, since you'll typically have higher dps and be able to take more damage (I'm simplifying things, but that's the gist). In Starfarer, the ships that can mount a Tachyon Lance sacrifice comparitively little for it.
The stock hulls that can mount a Tachyon Lance are already very strong defensively, and mounting a Tachyon Lance is both an offensive AND defensive choice. There simply is no better option for the OP. Perhaps in the final game, the balancing factor will be the scarcity of the weapon. Even still, increasing the firing rate and reducing the range to 3000 are fully justified, imho.
EDIT 2: I got a bit side-tracked there. Oops.
