Here are some test shots at 4000mm. This is the Reflex 2000mm with the Pentax A 2X-L TC attached.
These were (obviously) tripoded, and triggered from live view (no mirror slap) with the remote.
These are shots of Mt Olympus from my driveway, probably about 5 miles and several thousand vertical feet away.
Using live view, zoomed in a couple of times, I could visibly watch the heat distorting the image in and out. Also, our air, well it's not the cleanest in the nation.
So without further ado...
From top to bottom:
4000mm shot
4000mm 100% crop
4000mm 100% crop #2
31mm for reference.
Robin at 2000mm (atop the tree at the back of our yard)
Robin at 2000mm 100% crop.
(Please ignore the sensor dust :P )