I spent some time testing yesterday with the antenna replaced by a 50R load.
Rather that use an objective set of measurements such as an analyser at the IF output, I used the simpler technique of just listening. After all this is what most operators really notice!
Incidentally, not all the birdies seen on the P3 or other SDR attached to the IF output are audible at the receiver output and are possibly due to shortcomings of the SDR in the P3 or other SDR (SDR-IQ in my case).
Tuning from 144.00 to 144.500MHz, with the internal transverter, I recorded about 10 significant birdies. Some weak and some medium strength. There were no strong birdies.
With the Anglian selected as an external 144 to 28MHz transverter the number of birdies dropped to under half those with the internal transverter and many of those that remained dropped in level to barely noticeable. I have therefore decided that the Anglian is a better choice for Dxing. Well, I would, wouldn't I?
I suspect that the problem with the internal transverter is more down to lack of screening than anything else, although the extra IF output filtering incorporated in the Anglian also probably helps.
I plan to do some objective testing in due course.