I would guess that a dedicated external GPS would work a bit better though.
Definitely. For a start, most phones have Assisted GPS, which means they use the network location to "assist" with locating satellites to lock-on to. In flight mode, networking is not available and with the weak GPS signals coming thru small windows or gold-plated heated windscreens, the GPS sometimes never locks on.
An external GPS sensor like the XGPS-150 doesn't need any of that. It'll get a GPS position much more readily. It is also good for preventing i and a devices overheating in the sun if they are placed on the dashboard to get a good signal.