General response from Kevin Trenberth:
In general, these are good questions. I would not claim that GPSRO is a benchmark observing system but rather that it has the potential to be so, and its characteristics (vs IR and microwave) certainly make it complementary. But several issues remain to be resolved and a reference sonde system may be part of that resolution. Some of these questions can be answered from the BAMS paper on COSMIC coming out soon: a galley proof is available.