I was using the standard TX/LCD RX combo and antennas, except for the flight using the panel antenna. I don't want to suggest that the result was unacceptable, just that I've never got a packet during HP ascent, and lost the signal at about 2 miles during recovery on one flight. Still, have not lost a rocket, and continue to use the Eggfinder!
So, the subject of temperature effect at high alt is interesting. While the ambient temperature at altitude is well known, its affect on the internal bay temperature and batteries is not. Although altimeters such as the PerfectFlite do record the temperature, I haven't seen any posts on this. From what I have seen, the bay temperature is stable or actually goes up.
This would be a great subject on TRF; internal bay temperature and affect on LiPo batteries during high-alt flight. I consider high-alt to be 50K' and more, but it's still plenty cold at 20K'. Great Arduino based project with some temperature and voltage readings and a data logger.