Hi Norm,
I've also dealt with the AF limitations of all of my Pentax bodies since the DS over 10 years ago, and have upgraded bodies just to get the incremental improvements in AF performance that each has offered. When the K-1 was announced, I placed myself in the "probably won't get one" category as I assumed about $2500 in price, and there really isn't any "full frame advantage" for what I shoot (birds mostly). My "local" (70 mi away) B&M dealer knows what I shoot, and suggested I make the trip to try the K-1 despite my misgivings.
I made the trip, bringing my goto birding lens kit --FA*300/4.5 + F1.7x AFA (I have two of ea of these), my DFA 150-450, and my Tamron 28-75/2.8, FA50/1.4, and FA77 Ltd.
He let me use his own personal K-1 body, let me set it up however I liked, and let me shoot as much as I wanted in the store and outside in the parking lot. I brought my Asus Zen 15" laptop so I could play with the files there onsite. I alternated shooting with my K-3 and the K-1 with the FA*300+AFA and was surprised at how well the f7.7 max combo locked focus especially indoors compared to the K-3. Just chimping on the LCD, I could see that the K-1 was giving me better AF accuracy.
I was allowed to use an empty desk to play with the files on my laptop. Examining the images on the 4K laptop screen, I was pleased to see that critical AF lock was consistently more accurate with the K-1, especially in the lower light situations. I was shooting single shots, refocusing with AF each shot, even if I was shooting the same subject, as this is how usually I shoot when out in the field. The lower light AF performance is important to me because birds don't always choose the best lit branches to sit on, and often I'm shooting deep in the woods or in very shaded areas. I shoot opportunistically, so I need to be able to shoot in any lighting conditions that present themselves, and with an f7.7 max aperture, need all the help I can get from AF.
Also, with such a slow max aperture, high ISO comes into play more often than for most. With the K-3 I try to keep it under ISO 1000. With the K-1, I'm finding that 4000 takes about the same amount of work in PP to give me very good final images, and 5000-6400 might work as I'm still experimenting with some different techniques in PP.
I must admit that I don't shoot a lot of bursts, as I mostly shoot perching songbirds. I do always set up my shutter in Continuous High in case I want a burst, but burst speed is not a high priority for me, and 6.3fps in crop mode is easily good enough for my use -- and even 4fps in FF mode would not decrease my keeper rate.
Bottom line, the higher ISO capabilities plus the more versatile (lower light capable) consistently critically accurate AF made the difference, and I wrote a check and walked out with a brand new K-1. Another factor was the wider FOV VF, and the ability to change from FF to crop mode and back with a click of the dial. I've had enough situations where using a prime was a bit of a problem because I was too close, and stepping back wasn't an option because my subjects would spook.
Body weight difference is also not much of a problem for me though my aging joints do complain. I find myself picking spots that are easier to get to, so this is really not an issue. Also I'm not counting cost as an issue, though I realize that this is a high priority for many.
Bottom line, I'm getting more keepers than with previous bodies. I'm surprised at the higher percentage (maybe 20-30% better) of shots with acceptable or better feather detail because of the AF-S improvements in lower light and its decisiveness. I'm now able to get better IQ because I can shoot at higher ISO for faster shutter speeds and I can even stop the lens down for better resolution if I want.
I don't crop much for extra "reach". I generally shoot pretty close, which is why I prefer the 300mm+AFA which will mostly focus to @ 6ft as opposed to longer optics which typically have 10-13ft MFDs. I do shoot the K-1 in crop mode a lot because there's little sense to shooting in FF mode and capturing 20 or so MP that I'm going to crop out anyway. When I crop, 90% of the time it's very slightly for composition.
I'll include a few samples, but I've yet to develop a downsizing to 1400 pixels on the longside workflow that do the images much justice. Note that the first is at ISO 2500.
You've asked for samples showing IQ improvements in a number of posts, and I can understand this, but I would ask that you actually shoot a K-1 and compare it directly to what you currently shoot before stating that there would be no reason for a tele shooter to upgrade. It's fine to argue specs and try to imagine the differences in shooting experience and results, but it may be quite different in the actual shooting.
I'm very happy with my decision to get a K-1, and think you might actually be impressed if you tried one.
Scott