If you set the camera at 55, to ƒ/4.5 when you zoom in and out it will maintain the accurate shutter speed throughout if in AV mode.
K-5 and DA 55-300 PLM
Is it the best light weight option for both the K-1 and K-5? Absolutely.
All images taken with the K-5 (original).
You have to be aware of what you're doing... even though it "doesn't work correctly" compared to manual focus it's awesome. Compared to screw drive noise it's awesome. The AF speed is 7 times the speed of DA 55-300- WR at 55mm and twice as fast at 300mm... it is still best in class, and best for the K-5 despite it's limitations.
If you are going to have no options for changing your f-stop, 55mm 4.5 and 300 mm ƒ/6.3 are great all purpose options.
If you have the lens and a K-5, it won't take you more than a few minutes to figure it out.
If you don't, you'll probably just keep spouting half truths.
And the big bonus, if you upgrade your body, you're all set.