As much fun as that sounds, suddenly going from +1 Perception and combat dice, to +6's... the wording of Diagnostics is where it falls apart.
Core pg 257, Diagnostics
The Diagnostics power allows the sprite to evaluate the inner workings of an electronic device. The sprite can assist someone using or repairing the device with a Teamwork Test. The sprite makes a Simple Hardware + Level [Data Processing] test; if any hits are rolled, the character gets a +1 limit bonus, and each hit adds 1 die to the character’s dice pool to use or repair the item.
The reason why a Sprite can assist a Street Sam, is b/c they are actively using their cyber-arm, or their Smartgun.
Same goes for the Decker, who is actively using their Cyberdeck to perform Matrix actions.
But the Rigger isn't actively using their RCC in combat, what is actively being "used" is the drone they're remoted into. Which is why TMs can't just run a single Sprite on the RCC and hand out bonuses to the entire fleet of drones. You have to assign the Sprite to that individual drone for it to benefit.
So in the case of PI-Tac's, they aren't actively being "used" to provide those bonuses. They're gathering and disseminating data to the teammates, and the teammates working together through that data, amounts to dice bonuses. Knowing where your friends and enemies are, with sensory feed of the entire battlefield, equates to Perception and combat dice bonuses. The PI-Tac isn't "giving" you those bonuses, it's just giving you data.
So no... I would not think a PI-Tac benefits from Diagnostics. Sorry =)