@Sean
Maybe 250-270%, or at least just 220-230%?
That would compensate by being clearly superior to just two turns of firing, but would still feel 'fair compromise' against 300% nominal for three turns because of potentially skipping the cooling turn, balance considerations (which RC pointed at not making it 'default firing mode') and that small fluctuations-corrections are in the spirit of making good compromises (as opposed to big nerfs or dull 'mathematical' balance like simple scaling).
If taken to descriptive terms, it could be 'double the power and some more', leaving up to Piecewise to consider how to interpret it in each case, how much to go into detail.
@RC Well, now I am a little more reassured and relieved. There's reason your character rose to the position he holds
Anyway, I am just mentally comparing it to overcharge mode of the gauss rifle, taking it as a template. It is *not* the 'default firing mode' (which is your main concern here - correct me if I am wrong), but it is entirely so for in-game reasons; theoretically it allows for firing each turn (ehhh... I am not sure here, but in case of turn length relative *only* to its user, that stands correct, according to Battle of Hexbarax testing) and is several times more powerful than the ordinary shot (can't say for sure, but I recall it being charged for 5 or so seconds compared to 1,5 default, and it *can* overpower Civic Defenders longcoat and potentialy even damage a battlesuit plate, neither of which a single ordinary gauss round can do).
And finally, just as you, I feel that flat double the power for triple turn cost is (somewhat) underpowered and thus somewhat unfair. Double the power and a little bit on top may be not, however.