The wifi board is very light weight, and when build inside the shell it doesn't add to the air-drag everything external produces. It does use power (not sure how much) but i'm happy with the flight times i have now, from 100% to 15-20% gives me about 30 minutes.
I always fly my P3S in ATTI mode, so that may contribute to the lower power consumtion, since it doesn't actively airbreak when i let go of the sticks. Still i like the ability to turn it in GPS mode during flight to make it hover at one place, especially when it is already far away and almost out of sight.
All put together, i think the good points of having telemetry outweigh the bad, although my original idea was to remove the legs to get the true aerodynamic shape of the phantom. Also thought about adding real short ones to make landing a little easier, like this:
I will soon start a seperate topic about the complete mod, there is more to it you may think because of the limited space inside the Phantom 3 (S).