Hi Tom,
I don't want to take up too much space here with my response, as the thread is mostly about long lens options with the IQ/Mamiya system. I hope though it does provide some info that long lens options are avaialble in medium format and in some cases can hold their own (with of course limitations) against 35mm systems.
In response to your question, nope, haven't gone to the "dark side"...LOL, although I am using both the D800E and 645D currently for different purposes. The 100% crop I attached to this response is only partially indicative of what the Pentax 600mm 645 lens is capable of. The reason I say partially, is that the shot was taken at ISO 1000 due to low light conditions, which in itself is good on the 645D, but will indroduce some noise and loss of detail. Addtionally the reason I haven't completed and posted my evaluation of this lens's performance, is I'm finding that slight changes in how both the 645D as well as the lens is handled during shooting as related to techinique, can often yield widely variable results. This posted shot, which was solely taken as part of a test was where the lens was simply mounted on a Wimberbley side arm and it was in free rotation mode on both axis, not an ideal set-up for extracting maxium sharpness nor performance. A two point support where both the lens and 645D are independently supported (seperately) yields considerably greater sharpness...yet of course is impractical for moving subjects. So in essence, I purposely took this shot knowing that a number of important variables were not optimumly set (free floating axis while shooting, ISO 1000, subject slighting moving, no cable release, single support for lens only etc.). Of course the lens is manual focus.
So the short/brief answer to your question is the lens is capable of excellent, sharp professional results at f8 and beyond for use in very sizeable large format prints and when used at f5.6, is eminantly useful for decent-good sharpness and output with a fairly noticable but not tremendously significant drop in performance. Some limitations may be present when shooting at f5.6, depending what ones desired output and use of the files are going to be. At f8 and especially beyond, the resolving power and capability of the lens went well beyond expectations. Again this is from use of the lens on a cropped 645 body (the 645D). One though should not expect the extreme acuity of lenses such as the current Nikon Supertelephotos, but the pentax 600mm does hold it's own in a great many situations.
I'm also testing this lens with Pentax's 1.4x teleconverter under various conditions and when optimal, it's quite promising (for fairly critical use).
Results with proper lens/camera support along with impecable technique and shooting at low ISO is of course superior...but that would not have been real world shooting in this particular situation. These particular "Little ones" are constantly moving/heavily breathing and rarely sit still...LOL!
Dave (D&A)
(100% Crop of the full frame image that was posted in a previous post, with light to moderate non-optimal sharpening applied)