You can still claim objectives in your turn 6 even if you go first. Then your Piranha only has to whether 1 turn of fire and that shouldn't be a big deal by turn 6 if you've been doing your job and properly removing enemy heavy weapons.
Also I have recently come to realize that the free Gun Drones on Piranha are worth more than even normal gun Drone squadrons and are reason enough to take Piranha in and of themselves.
First of all they don't take up an FOC slot. Which is important because you could potentially have so many extra units on the board for fire dilution.
Secondly they can be delivered long distances by the Piranha either 12" or 24". Thats handy when you really need to get some extra firepower to a hotspot but then need your fusion guns elsewhere.
Third they can open top assault 20". This has saved my units countless times from CC and/or tied up many many heavy weapon squads.
Fourth and finally and most important aspect is they are worth no VP's. This makes them the ultimate roadblack/harrasser unit since they are completely expendable(the way drones should be). No other unit can be thrown at the enemy with such abandon as free drones since everything else will give up VP's.
The fact that they are I4 and probably the best CC unit the Tau have(yeah I know about the Kroot but they get no armor save
) means they are ideally suited to the role. Toss in their ability to pin and use their assault weapons as well and things get even better in the role of sacrificial roadblock. At this point JSJ is just a bonus for moving them into the face of the enemy sooner.
And what does a regular drone squad get that they don't? The option to deep strike, and the status of a scoring unit. Now deep striking is not always reliable for many reasons. First the rules for the scenario have to allow it. Second it requires a successful reserve roll. And finally they can scatter onto enemy units, immpassable terrain, or off the table and they are dead. As far as scoring status goes, it's not like you are actually spending points on non scoring units, just geting free non scoring units in addition to the ultimate scoring unit...the Piranha.
So see Piranha drones are really worth more than their standard FOC cousins since they have so many usefull and unique advantages. Factor in this fact when deciding how much a Piranha is really costing you and what you really get for your points. BTW don't spend 5 points on decoy launchers they rarely save the Piranha and just add cost that could be spent better elswhere. Now your Piranhas become even cheaper at just 70 points.
Now before someone points out that the drones on Devilfish are free too, I'd like to point out a few things about DFish.
First they are dedicated and non scoring. Also they are 85 points bare minimum upgrades. And last but not least, their drones max out at squad size 2 and can't open top assault, nor be delivered 24". Thus they do not make as effective roadblocks for all those reasons as they will usually not hold out even 1 round of combat.
Piranha drones on the other hand can have a squad size up to 10 but even a squad of 4 drones from 2 Piranhas makes a much more effective CC speedbump.
So yes Piranhas are competitive and not jsut for their effecive fusion guns.