Simon:
I have a Gitzo CF tripod; I screw the spikes in firmly as far as they go (25 mm), and they are very solid.
If the machinist hadn't cut too much off my longer bolts, I was going to put a washer and two half-height nuts at the bottom of the thread - this would help to keep the bottom of the tripod legs from pushing into soft ground. But being all plastic and CF, they wash out easily.
Glenn