GPS-Logit App on an inexpensive Android phone and the excellent waterproof arm bag by H2OAudio. Hands down the best for a phone in this use in every way.
It has big colour screen, race countdown and race timing mode. Audio feedback speech of countdowns and speeds. And it also does Alphas!

Combine it with a Bluetooth Ublox M8 GPS (DIY only) and you have something that can also be approved for GPSTC, but it will do pretty much as well as any other non GPSTC approved GPS just using the built in GPS.
The main disadvantage is battery life. With most inexpensive phones it is only about 2 hours, some a bit more. How long does a race last?
gps-speed.com/gpslogit.html