The focus difficulties that I've experienced with my DA300 may have been specific to the lens; it is being repaired under warranty for focus mechanism issues. Others may confirm or have other experiences.
The focus speed of the 150-500 is faster than the DA300. The increased magnification of the subject makes the large focus points of the K5 more effective, where a subject at any distance is a fraction of the center focus point at 300 mm, making it difficult to focus on the contrast point that you want in a busy scene.
I used someone elses DA300 to try out and it seemed more decisive and accurate, so it may have been my copy.
The Q with adapter is fine for static targets, or nearly so, on a tripod. If that fits your style of shooting, then it may work for you. I use a manual focus 400mm on a monopod with gimbal, and it worked well. Anything you choose will have limits, it simply is a matter of figuring out what you will be shooting, at what distance, in what light, if they are static or moving, and what results you want. Then you spend the money