Obviously the versitility of the P5 would mean I can use it with any future cameras I buy, as well as on the 40D with or without the battery grip. However I am planning on using my new ballhead for some tracked long exposure astrophotos, using exposures of up to 10 minutes and wouldn't want any slipping from the camera plate in the cold temperatures. Would the P5 be up to the job or would you recommend I go with a model specific plate just to be sure?
I've had a Wimberley P-5 mounted on a Canon 40D and a Canon EOS-1v for several years. If you get it lined up and very tight in the beginning, it will never slip when mounted on a tripod. I trust my P-5 plates so much now, that I never think about them anymore. I have never had any slippage of any kind, and I have never had to re-tighten them either. They just work, and work very well. I can highly recommend them from my experience.