It's possible that they don't want a higher tow speed due to increased noise from the TPL passing through the water.
Another reason for the wing, it isolates the towfish from the towcable. Harmonics from the cable has a significant effect.
Hydrodynamic drag increases as a square function of velocity, so isolation of the towfish connections is also very important.
If the towfish is isolated, I would expect 5 to 7kts without issue...you can tell pretty quickly.
Again, commercially, time is money, and given that pinger locators are seldom used, I would expect that the technology, and the logistics of using them, may not be the most current.