Yeah so my proposition was not necessarily to make the t3 subs really more powerful (hence the title is misleading, I just edited it to correct it) but to make the tactical missile they launch more "inside" of the gameplay.
For instance making the subs shooting in burst rather than 1 missile every x seconds like they do actually (to surprise an opponent). Or reducing the rate of launch but making missile more powerful in order to "snipe" stuff. By doing this trick maybe the DPS would be lowered in the end for balance purpose (it could be one possibility), but they will have some usefulness within the gameplay (which they lack now). I also like the idea from @H-master (it can be another alternative if it is technically possible).
The reason of all this is because I honestly never saw in any game those missiles being useful for anything (and I am a regular player since years, frequently playing navy). I think such proposition could really enrich the gameplay without making the unit too powerful if the changes are appropriately made.
This is a last attempt to give some pros to this feature ^^