You will have GPS as long as you have cell coverage, go out of cell signal range and you're hosed. PDAs also have GPS /map software thats useful when you don't have cell coverage
I think most people put more usefulness on the BT feature until they use it. Environmentally the only thing thats going to change quickly past initial set up for the day is the wind and its not really that big of a deal to have to manually input that reading as you put updated range info into the pda. The most you are looking at saving time on is hourly updating those other environmentals , if they change much at all from hour to hour. I've also seen people spend time trying to get the two devices to talk to one another than it would take to just do it manually.As for speed/performance, I don't know what you hope to gain as even an old generation ipod can kick out firing solutions incredibly quick. Wifi, pdas have it too but unless you're in wifi range and or have a need to check your email or surf the net, its not gonna help you ring steel.
Don't get me wrong, you can probably make that set up work for you if the kestrel will talk to the nexus. I'd check that in store with your kestrel before you buy the nexus. But I think your putting more emphasis on the nexus and the BT link than it's worth IMO.
