Based on the last two answers I may be missing the point of the question here, but instead of worrying about aperture, your question more directly relates to how do I track action.
Tracking abilities I would split into 3 categories.
- The Camera
- The Lens
Part 1: The Camera
Some cameras just wont do tracking particularly well. You really need one that can do continuous auto-focus (not all do). This is the AI Servo mode if you're in Canon land, or on Nikon there is a C/S/M setting that does this (you want C for Continuous). I'm afraid I can't comment on Pentax, but refer to your manual. So set your camera to continuously focus.
The next thing to check is to ensure it is using all available AF points. Again, this varies by camera, but you may have a diamond of 7 or 9, or be lucky enough to have more - 19, 51, 61.....depending on make and model of camera. Just make sure all of them are enabled.
Also, think about the metering mode you choose. for things against the ground, or landscapes, you may want evaluative or full metering, but for a bird or aircraft in the sky, choose centre-weighted or spot metering.
Finally, choose your shooting mode appropriately. For action shots you will want shutter priority mode. I think Pentax use the mnemonic "Tv", as do Canon. Nikon and others just use "S". Then set a shutter speed appropriate to your action. Fast moving = 1/1000th or more. Slow subjects = 1/250th maybe? The benefit of your fast lens will be felt as you can keep the ISO low whilst still getting correctly exposed shots.
Part 2: The Lens
I'm not sure this pertains to you as its about image stabilisation, and I think the Pentax uses in body stabilisation. But in the Canon/Nikon world long lenses with IS/VR modes also have a stabiliser "mode". If you have this, ensure the mode is set correctly for your shooting type. Mode 1 is generally for overall use, handholding, normal shooting. Mode 2 usually only tries to correct vertically, so is useful for panning. Perhaps in the body of the Pentax you can select the mode too??
Also, the AF of the lens must be sufficiently fast to keep up with the constant refocussing by the camera body. But the lens you suggest has an SDM motor which seems to be well liked and fast.
Part 3: You
Practice your panning. Practice following targets across the ground and sky. It doesn't have to be in the dead centre of the frame but there's nothing worse than a raft of photos of your favourite car, plane, bird, etc with it's back end cut off! You may find a change of stance helps, or to hold the lens differently, or use a Monopod. Also, I would not tend to keep both eyes open as when you are tracking action, it only serves to distract you. Concentrate on whats in the viewfinder as thats what the camera sees.
I hope that helps?