The cheapo effective 1,650 mm 5.6 tele (Q + adapter + Tamron 70-300 and other lenses)

Over in Pentax Forums there's a recent thread that explores this subject. In theory, you are correct. On the other hand, in practice the best balance between resolution and sharpness was found to be around f/4 and f/5.6 depending on the lens tested.

An interesting read...

The OP is "hnikesch" ...worth the effort to find.

Cheers...M