I have Ultimate GPS and KIWI OBD2 WIFI. Last time I was at the track, Ultimate GPS worked at 3Hz although it is spec'ed to be 5Hz. Kinda disappointed at that, but it is still better than iPhone's internal GPS unit.
KIWI OBD2 WIFI works decently. It is suggested to but the phone in airplane mode (and of course turn on the wifi) and have no background apps trying to fetch data. Otherwise the data you get from KIWI is sometimes wrong. Also, unfortunately, you may not get all the data you are looking for. It depends on the car, and what the car wants to send out. For example, I get to see revs and throttle input in my car, but not brakes.
2011 MINI Cooper S
previous cars: E92 M3, Z4MC, Z4 Roadster, E36 328 Sedan