Some of your DA lenses might already work on full frame:
https://www.pentaxforums.com/forums/10-pentax-slr-lens-discussion/31629-da-le...ts-thread.html
And third part lenses are mostly full frame as well:
https://www.pentaxforums.com/forums/10-pentax-slr-lens-discussion/183420-curr...me-lenses.html
Either way, not really. The thing is, full frame is bigger and you get a wider field of view with the same focal length. So if you have a 35mm lens on crop, its not really wide angle, but on full frame, it is wide. I think this is the main draw to full frame. The other thing is shallow DoF. Since the sensor is bigger, the DoF looks comparatively more shallow.
So I would say if you do landscape photography or interior photos where you want wide angles, FF might have an advantage, even though these days you can find ultra wide lenses for crop sensors as well. And if you do portrait or abstract photography where you want really shallow DoF, then FF might have an advantage.
The benefits of noise performance and diffraction are less and less prominent, because technology is catching up and reaching its limits. But since FF cameras are usually expensive and top tier, they often have other features like fast burst rate or improved AF, metering. So when the FF comes, don't judge it only by sensor, keep in mind the other features.
All that being said, the K-3 is, as many say, the best crop sensor DSLR available. So I really doubt you would regret buying it, especially with some of the nice discounts we have seen recently. Of course, some are hoping that the K-3's price will go down when/if the FF is announced, but who knows. Maybe the price will go up, since new people will join Pentax now that it has an upgrade path
Oh, and I am sure if/when an FF comes, it will also allow a crop mode to use crop lenses. Or at least you can crop photos in post production. Then there is no difference, except some area of the sensor that is simply not being used with those lenses.
Last edited by Na Horuk; 09-23-2014 at 09:31 AM.