If you own a Penax, why not find a used or new PK compatible lens before switching brands? Every dollar you spend on another body is less money towards the lens.
Since you have the more than adequate 55-300, I'd recommend looking for a prime in the 400-500mm range, though I'll admit the Pentax 150-450mm is a tempting option.
Another option is a mrror(refractor) lens. My local camera store has a manual focus Pentax-A 500 mm mirror for sale, I'd buy that before dropping $2k on something else, IMO. On the Single in challenge this month, at least 3 people are using 500mm Mirrors, and their photos look far better than my M42 Kalimar (Minolta)500mm fixed at f8 with a PK adapter, though now I am finally getting some sharpness after using it ever day.
[IMG]
[/IMG]