Apple don't disclose any specs on the GPS in the IPad. On some of the computer tech forums there is a lot of discussion about them using mobile phone tower triangulation rather than full GPS. There is also an issue about potential antennae screening from the cockpit. My Garmin 495 gets dodgy reception if its on the seat beside me rather than the glareshield. The iPad (I believe) will take a gps signal from a bluetooth gps engine. You might consider this.
One of the reasons I bought the 495 was for the synthetic instrument panel. I've just had my third vac pump failure. Only one was in IMC (just exiting cloud), but I like the idea of the backup.