I am pleased to report that the problems I reported when using the Elecraft K3 with a 2m transverter requiring a high power IF input have been fixed. I received a beta test firmware version 3.00 in my mail this morning from Wayne at Elecraft. The K3 will now remember that my transverter needs 4W of drive out of the antenna socket, instead of a few milliwatts out of the non-existent KXV3 transverter interface. And it does not prevent me from transmitting when I set the receiver to 145.700MHz or above.
Fans of Elecraft will point out that no other radio manufacturers would provide a fix for a reported issue within two weeks, and they may well be right. But I still think it would have been better if the bugs had not existed in the first place.