I was mostly referring to really old 5k range tach lance, not sure which game version it was. I think it was only 0.5 efficiency and not too damaging, but the fact that you were pretty much always threatened without notice made it quite OP.
I really wanted to try that out just to see how overpowered it really was. I read that it had double range and DPS (but no chain EMP) compared to 0.53 version. Since it is a beam that probably hit for soft flux, I still have doubts that it was overpowered that others claim. I cannot find any working links here for versions before 0.53.
The original Tachyon Lance was an awesome game-changing weapon - but a large part of its (relative) power was due to the different combat environment.
Back then, you had a very small limit on deployment points that increased with nav nodes you controlled - this was the original value of the comm relay nav nodes, and back then they were the priority target, because each one you controlled meant an extra 30 deployment points of ships you could put on the field. And since there were no CR costs, and thus no reason not to deploy everything you could, gaining control of a comm relay could make or break the battle.
With no ship systems or player skills, the fastest things around were fighters; this lead to a combat paradigm where you typically sent fighters or frigates out in the first wave to capture nav points, then followed up with heavier ships once you could afford to field them.
The original Tachyon Lance, then, meant guaranteed field superiority. A few wings of fighters backed by even a single Sunder with a TL could completely wipe out the enemy's fighters (thanks in part to another feature of the original TL: its beam wasn't stopped by fighters or missiles, and could damage as many such targets as you could line up). At which point you could deploy your heavier ships, and the enemy couldn't, and that was pretty much that.
It wouldn't be quite as OP now as it was then, since comm nodes are now worthless, there are frigates and destroyers that can take a TL hit or two and still get around faster than most fighter wings, and - while you would still be able to get crushing focused firepower by deploying a large number of 5k range Tachyon Lances - doing so would now have a rather significant cost in terms of CR and supplies.
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That said, I'm personally pretty happy with the current TL; it can't snipe, but it's otherwise a good weapon.
(Current state of plasma cannon, though, needs some help. Flux cost is too high; might be okay if it wasn't doing that three shot burst thing? Maybe we can get back the old charge-up plasma cannon; that would be fun...)
