Originally posted by Des I feel a bit the same way Kevin. I think there would be a noticeable gain in resolution with say the K-1 and DFA 28-105, but it would significantly lighten your wallet and weigh down your bag! And while the DFA 28-105 seems to be a fine lens, you'd miss the reach of the 18-135.
If I may make a suggestion, a K-70 or a K-P might be a better upgrade from the K-50, to go with your K-3ii. I'm now regretting I didn't buy a K-P when Pentax Australia had them on sale for $A990 a couple of months back, especially as the next flagship model seems to be still some way off.
One thing about staying with APS-C is that there is a wider range of current lenses available, especially at the wide end - e.g. no FF counterpart of the wonderful DA 15 or 20-40, or of the DA 12-24. And there isn't really a FF equivalent of the 18-135 (28-200) or the 16-85 (24-125) in the current lens range. Nor is there a counterpart of the DA 55-300 PLM. And if there were, they'd be bigger, heavier and probably a lot more expensive.
The full frame bodies are really nice but be sure to identify what it is that you want to improve on and at what cost would you do it. And the cost isn't just for the gear but the cost of carrying around a bigger body and bigger lenses and worry about your gear's safety. I have a K-1ii and also a 645D in addition to my K-3. I still shoot with the K-3 and 18-135 (as well as other lenses) on a regular basis because it's often the most appropriate tool for the job.
If my photo mission is to try and get that magazine or calendar cover of a spectacular sunrise, I'll probably bring a bigger kit if the location is compatible. But much of the time I'm cycling, hiking, skiing, or in an urban exploration situation and I need to keep the camera kit compact and have it attract less attention. In those cases the K-3 & 18-135 is often the choice because even if it isn't my best gear, it's plenty good enough to not be the limiting factor most of the time. Having the smaller camera along and ready has allowed me to get a lot of shots I wouldn't otherwise have been able to get.
Also, I never got an equivalent zoom for full frame because I already have the K-3 & 18-135 setup. For full frame I got more specific primes and higher grade zooms for when the shoot calls for it.
I carried the K-3/18-135 combo on a nice little hike yesterday. It was all I needed!