I tracked this lens on Ebay as well, but after an exchange with the seller decided not to take the risk. Looks like you may well have a real bargain on your hands.
My previous experience with the Sigma 500mm was that it did need to be stopped down to get really sharp images so I expect that is the same with the 800mm

Three lovely images, and I agree with previous comments that the third one is the standout.
Technical quality on all three is excellent so it answers your question on the pairing of lens and TC with a resounding yes.

A little jealous of the first image - my pear trees (conference and desert) have decided to take a year off! Probably felt they worked too hard last year when we had a bumper crop

Great set of images. I think my set of 'M' series lenses (including the zooms) produce some outstanding results and so much more compact than modern equivalents

Nice set Graham, considering that Clumber Park is only 25 miles from me I really should visit. As ever though it is often the attractions on your doorstep that you fail to visit.

Agreed.

On my laptop screen the IR converted image has some areas of blue which are a little disconcerting

Nice work though

Some great images Steve. Like David I really like the shot of the Verbena

Great work. I particularly like 2 and 3 for the silhouetted buildings
Thanks for sharing

Thank you all for the kind comments and observations. I have been a little lazy over the years about posting in the forum, something I will try to rectify!!

Picking up on Steve's (Lubbyman) comment my use of TAv on the K3iii is not limited to macro / close up work - I tend to use the setting for most of my bird photography as well. A definite bonus of the high iso performance on modern cameras.

A hot afternoon and feeling a little lethargic so I thought to try and liven myself up by dragging out one of my manual lenses - today's choice being the Pentax-A 70-210mm F/4. This lens was a firm favourite in years gone by particularly when paired with my Pentax Super A.
While I still have the Super A, today's images were taken with a K3iii.
Camera settings were TAv (1/1250 at F/6.3) Auto iso. Images processed in Lightroom and Topaz

1. Red Admiral



2.Painted Lady - 1



3. Painted Lady - 2



4. Comma



5. Green Veined White



6. Japanese Anemone



7. Poppy



The butterfly pictures are best viewed large
Thanks for looking

I really like the muted / pastel colours. When viewed large, the detail on the Bee in the first image is brilliant.