I remembered I had some pieces of HIPS in one of my project bags in the attic so I went to fetch it to test the drilling process out. Any potential concerns I may have had were completely unfounded. Using a spade bit with the plasticard clamped in place, holes were drilled with extreme ease. I used a power drill/driver for this which allows me to go pretty slowly. The holes came out superbly well. As with any cut using plasticard (drilling or scoring/cutting) there's always a slight lip around the edges. This can easily be removed with a decent medium/high grit sandpaper.
Back to work tomorrow
Edit: I have removed the screws from the throttle fascia and the panels still seem to be held in-place. I now don't have to concern myself with drilling holes for those. That was one of the reasons for using a slightly thicker material in the first place - I like things to look as seamless as possible.