I also have the older HD WR version, for many years now, and have hung on to it. AF has been good enough, and actually improved for speed and sureness noticeably with the KP! If you don't go to extremes from near to far distance back and forth, this also allows AF to be a lot quicker. It and the new PLM version each have their own advantages.
A main reasons for less AF speed in the old version is a longer rotation focus throw. But this can be an advantage when it comes to manual focus, including the quick-shift AF manual focus touchup. Having more rotation range allows for easier manual focus fine adjustment. The old manual focus lenses typically have more rotation range than is typical for AF lenses.
Another advantage for this older lens is in not being of internal focusing design, but has instead a standard focus mechanism. The internal focus technology can help with focus speed, but also tends to introduce what is called "focus breathing" which will shorten the effective focal length at shorter shooting distances. So if you say creep up to get fairly close to a bird or other small wildlife, and are say shooting at 300mm, the resultant size of that bird in your picture will be larger with your older lens than it would be with the newer one. If you'd be happy enough with the smaller size the new lens could provide at 300mm, with your older lens you could shoot at less than 300mm to get the same size, which should improve image quality. Zoom lenses tend to be not at their very best when racked all the way out.
The upgrades of this optical formula by adding HD coatings and WR construction were important upgrades. I used to have the previous version, which was slightly damaged but only to filter threads so filters could not be used. Upon getting the HD WR replacement, I was surprised to find improvement in image quality was noticeable due to the HD coatings! Better contrast and detail.
Last edited by mikesbike; 09-16-2021 at 07:23 PM.