Well, the problem with my QCX was the 560pF capacitors C25 / C26 in the low pass filter.
I have replaced these with new components of the same value, and now, having gently tweaked the turns spacing on the toroids, I get over 2.5W with a 12v supply, and almost 5W with a 15v supply
The symptoms are discussed on the qrpLabs groups.io forum, in a thread titled “A word about QRP Labs capacitors and toroids”, started by Hans, the designer of the QCX.
It looks as though there might have been a bad batch of caps affecting the 20m and 30m low pass filters (though Hans is still investigating).
Typical symptom is that the power output creeps up over a minute or two with key down. I think this is because (a) the capacitors are a bit lossy and so warm up when in use, and (b) they are not NP0 as they should be eg the value reduces with temperature increase (I can demonstrate this with the removed components), shifting the cut off frequency of the filter.
Anyway, an interesting diversion, an easy fix, and a nice end product!