If the optics are fine, then you'll be hard pressed to see any difference. Just a very small softening - at 100%, and you must look at two of the same subject with/without the TC in place - not significant at all. It's one of their best TC's, although MF only.
FYI on compatibility, since you've got the F* 300/4.5 (from B& H website) - not sure if you saw this:
_________________________________
The Pentax
SMC-A 1.4x L Rear Converter (
also known as a tele-extender) is desigend (no proofreading on this one I guess...
) to fit on select Pentax 35mm "long" lenses.
<snip>
Note: Due to the high-quality design of this converter (
5 elements/5 groups), the teleconverter will
only fit and work with a select range of lenses. Only the following lenses are compatible:
- SMC-A* 300mm f/2.8(*)
- SMC-A* 400mm f/2.8(*)
- SMC-A 500mm f/4.5
- SMC-F* 600mm f/4(*)
- SMC-FA* 600mm f/4
- SMC-A* 600mm f/5.6(*)
- SMC-P 1000mm f/8
- SMC-A 1200mm f/8
- SMC-F* 250-600mm f/5.6
- SMC-FA* 250-600mm f/5.6
- SMC-P 400-600mm f/8-12
- SMC-F* 250-600mm f/5.6
- SMC-A Macro 200mm f/4
(*) Note: Fits, but better performance can be achieved with other versions.
_______________________________________
Note that my FA* 300/2.8 is not listed, but it works just fine.
Cheers,
Marc