I've been using a wi fi only i pad mini for just over a year, with SD installed. I use a Garmin Glo GPS receiver, and have found that combo works fine. Never had any battery time issues, as slam has already stated. The only downside is remembering to take it out of the aircraft, as it is only the size of a Swan Vestas matchbox - remember them - and it can be easily missed on the combing!
My flying buddy uses a 3G i pad with a sim card on SD, and he VERY occasionally has a GPS reception problem, particularly at higher altitudes. I'm not an expert in these dark arts, but a bit of research on t'interweb implies that earlier i pads internal GPS receivers also used the GSM mobile network to improve their performance. I appreciate that this may not be the case with the current generation of 'i tech'.