Thank you everyone for your contributions. Since starting this thread I have managed to acquire a Sigma 100-300mm f4 to test against three other options. The lenses tested are as follows:
F*300 f4
Sigma 100-300mm f4
Sigma 150-500mm (HSM with OS - note OS was switched off)
Sigma 50-500mm (non HSM)
Testing was on a tripod with a target about 6m away. I tested each lens wide open and at f8. ISO 200 on the K3. JPEGs were generated within camera and then cropped at centre. Results are as follows.[COLOR="Silver"]
---------- Post added 17-09-17 at 07:44 PM ----------
F*300 f4 + Tamron 1.4X TC
I ranked this #2 in centre and #1 at the border
Open
F8
---------- Post added 17-09-17 at 07:53 PM ----------
Sigma 100-300mm f4 + Tamron 1.4X TC
I ranked this #1 in centre and #4 at the border
Open
F8
Sigma 150-500mm (HSM with OS - note OS was switched off)
I ranked this #3 in centre and #2 at the border
Open
F8
Sigma 50-500mm (non HSM)
I ranked this #4 in centre and #3 at the border
Open
F8