OK so I have done a bit of a test on my K5.
Q1. Is the DA much softer wide open at 300mm?
Q2. Is the DA better at ~250mm compared to 300mm so that there is little advantage to 300mm?
Test subject: the Vardre, a local hill, site of an ancient monument. Long distance = infinity focus or thereabouts. Method: tripod mounted, manually focused using 10x live view, shots taken using 2 second timer to minimise vibration.JPG files exported from original RAW using LR3, same default settings only, just equalisation of exposure or WB if necessary. Image was cropped to reduce file size, then 100% crops using Faststone.
While MF always has the possibilities of error (and on my lens it had probably never been used and was a bit sticky and twitchy) and the long distance the possibility of atmospherics to degrade results and produce inconsistencies, the pics are consistent with my experience of the lens generally and you may judge them for yourself. 300mm first three crops, 260mm next three.last crop is using a tamron 350mm preset lens just for comparison. This is a
sharp lens IMO but you can see that the DA is actually pulling more detail at 300mm than the tamron at 350mm.