My Ipad mini works fine with the built-in GPS for navigation purposes.
Evaluating start roll performance/landing distances however, requires a better precision and higher refresh rate as the internal GPS data are too sketchy, some seconds are missing etc..An external GPS would be needed for higher precision then. 
It worked fine for me to set the position update rate on my Garmin 496 to 5/s or to the lowest spatial resolution respectively. But accessing the data from the Garmin is cumbersome while getting logs out of SD and displaying them or exporting them as GPX is very comfortable
So, is it possible to set the data refresh rate for an external GPS to a  higher level than 1/sec for bluetooth connection as well as direct plug in Solutions? Can SD handle resulting larger log files if a GPS position is written to the file eg twice a second?

SkyDemon handles GPS refresh rates of up to 10hz just fine (I haven't tested any higher).

Flight log files are still recorded with data every 5 seconds, though. That's just the resolution we decided on, as a good balance between file size and spatial resolution.