Some food for thought:
Mounted to the Pentax 1.4x converter, essentially all of the DA lenses should work well (except perhaps for some slow zooms in dim light). With the 1.4x, you'll have the use of the lenses pretty much as originally intended in terms of field of view/magnification. Of course, you'll lose one stop of light when full open, but that will be counteracted with the larger sensor, netting a DOF about the same as what is seen with lens only on APSC.
Not only does this not really work the way you are assuming, it will never, ever, produce as good a quality image as simply using the crop mode option on the full frame (or any of the recent APSc models, for instance) - which yields a relatively similar FoV.
This is a classic example of how two wrongs make for doubly bad results. All I can say is, Pentax is probably looking at all the on-line exuberance for misusing optics and evaluating whether allowing unfettered freedom to produce bad images is a good idea.
I posed a question to the forum almost two years ago, asking if a teleconverter was really better than a straight crop with today's equipment. I believe this was before Pentax's new 1.4x TC was available.
The answers surprised me. Not only did many feel that a good lens still out-resolved the sensor by enough margin to overcome the quality drop-off from a TC, but they pointed out the advantage of seeing the full image in the viewfinder rather than a shrunken version, making focus easier and more accurate for example.
Would this be as good as lenses purposefully designed for FF? Of course not. Would it be good enough to get you by if your budget didn't include replacing every lens you own? Possibly.
http://www.dpreview.com/forums/post/52503219
The drop in quality will be relative to the IQ of the lens in use.
So though an f2.8 lens will deliver F4 FF the IQ will be in line with It's original F2.8 open aperture
Hence the 16-50 will be @ peak MTF @f5.6 but will deliver higher IQ than Either cropping or an F5.6 ff Kit lens
In general High IQ lens require 1 stop closure to achieve decent MTF whereas consumer lens require 2 stop closure
So based on this (which stands the test of time)
a FF kit lens will be at the same IQ as the DA* + converter (long end) at around F11 Vs the da* f5.6
a FF FA* f.28 will be at peak @ around F4
So IMO the da + HD converter will match any FF offering at the following
da* 16-50 = 24 -70 F5.6
da* 50-135 = 75-200 F5.6
da* 200 = 300 f5.6
da* 300 = 420 f8
and for reference some FF lens and where I think their IQ MTF lies
Sigma 28-70 F2.8 = f5.6
Pentax fa* 27-70 F2.8 = f4
Sigma 400mm Telemacro = f5.6
Sigma Bigma = f9.5
So IMO the da line + converter will deliver IQ inline with midsumer FF offerings which I believe is a far cry from
"it will never, ever, produce as good a quality image as simply using the crop mode option on the full frame"
I can only assume JNR either has no decent converters or glass combinations.
HQ converter + HQ lens = HQ results
HQ converter + soso lens = poor results
soso converter + HQ lens = poor results
soso converter + sosos lens = appalling results
crop + soso lens = poor results
crop + HQ lens = marginal (good) results
what crop mode in camera give you is the ability to get your speed back at the cost of resolution, a converter gives your the resolution at the cost of speed.
Both options are good and having both is best.
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