I had an unplanned early morning opportunity today to shoot the 14 and under portion of the wakeboarding national championships, and initially took only the D500 with Tamron 150-600 (bootin' and shootin'). It performed great, far better than the 55-300/Nikon combo
A couple of hours after returning home I went back with the K3III and 150-450. It too performed great. But that 600mm range was a definite advantage for the Nikon. I wished I could have filled the frame a bit more with the Pentax when the boarders were at the farthest end of the course.
Not having my 1.7 TC with me I can't say for 100% it might not have been just as good optically at an effective 630mm with it, but I suspect not. If I can get out tomorrow morning I plan to confirm that. But at the moment that used sub-$1000 Tamron has exceeded expectations and I'd recommend it if you're on a budget. Even if you're NOT on a budget for that matter.