Until I broke it, my K5IIs was the best Pentax I have ever owned. Just loved it!
However...I can relate to your AF complaint. I found that using the back button for AF greatly improved my AF speed and accuracy. How, I don't know, but it made a real and significant difference.
If you want the ultimate Pentax get the K1. I'll repeat this, though I have posted it all over the place..it is worth posting again.
I have been shooting Bigma 50-500s for almost 10 years...from the K10D forward on the flagship bodies. The Bigma is a big ol' slow lens and very hard to focus in low light. In fact, it hunts like a blind hound. The K5IIs was the best body so far, but still missed many shots due to slow to lock focus...or complete failure to lock.
So I ordered the K1 to replace my broken K5IIs......hoping for some improvement in AF and the long list of other outstanding features it offers...like superb high ISO results.
First lens to go on it was the Bigma. WOW! I couldn't believe it! Had to look again at the "Pentax" on the front! It is amazing...fast and accurate like no other Pentax before it! No hunting, no waiting, it locks and you fire and you have your shot...low light or not it gets 'er done...and fast!
The biggest difference for me is not better IQ, the K5IIs is excellent, it is the fast AF and that high ISO ability!
Now.....with a fast lens like my new Tamron 70-200 2.8, the focus speed is downright scary for an old guy! Makes my head spin!
The K5IIs will work fine for you with fast glass....it did for me..try that back button. But if you want a real thrill...get the K1!
It was just turning dark...around 8:30 pm. I saw her at the squirrel feeder out my office windows. Picked up the K1+Bigma.....locked focus instantly...fired.....there's that thrill again!
1/80 handheld (New SR is fantastic!) f7.1 @ 240mm ISO 12,800...no noise reduction!
Don't sell the K5IIs short......here is one from my broken K5IIs shot MF about the same time of day.
1/60 handheld (Great SR too!) f6.3 @ 200mm ISO 6400
Best Regards!
Pentax has your needs......you have to do the rest!
Rupert