Hi I use a GT 31 and put it in the back of my vest (neoprene) I use GPS arpro and everything works okay as far as i can tell.

Only thing i would like to know is if I can check how many spikes have been detected/deleted by the software, if this is 

a high number then I would go back to using the aquapack attached to my arm.

Sometime ago timekeeper Andrew Daff advised me to wear the GT-31 with maximum sky visibility. There was some doubt about my alpha speeds and he checked some of my uploaded tracks. In my tracks, he found lots of high SDOP values, causing the GPSR filters to reject them. The reason was that the bag with my GT-31 sometimes was hanging under my arm instead of on top (inadvertenly, after maneuvers, waterstarts etc.). So I gues that wearing under your suit (=no sky visibility) can produce the same inaccuracies, not only spikes.

Here an excerpt from his explanation:

"SDOP is the name we give to the accuracy data from the GPS system. It stands for Speed Dilution Of Precision. When the numbers go high it means that the data is very unreliable and indicates possible large errors and other issues.  The bag hanging under your arm is exactly the sort of thing that commonly causes this problem.  ...  If you can keep the GPS facing up on the upper arm it really helps a lot."