Since iOS 8 the performance of GPS navigation on the iPhone is highly dependent on the programmers skill. There appears to be a change in time slicing in the system, which may cause some apps encounter GPS dropouts. Most VFR navigation apps seem to programmed more thoroughly then street navigation apps, so I would not worry about. These dropout you will see also with external GPS, maybe masked a bit by the external GPS buffering, but the trouble is multitasking in the device, not the GPS.
When you encounter GPS dropouts you will face two situations: the moment the GPS picks up the next satellite transmission it is not far from the last one - in that case you only have a small dropout, but if the next signal is not within dead reckon distance, the iPhone needs to get a new first fix - which will last longer without mobile net (remember it is A-GPS and the assisted part uses mobile net as well as wlan sources).
Last, guess why used airboxes are so cheap ;-) - their value is not much given the iDevices world.