I nearly bought 75mm Timberdrives, but then had a think about it. Theres 50mm of the pre-drilled 'ladder' to go through first (or a bit more if going in at a bit of an angle, square of hypotenuse and all that), you want a good 'bite' into the log, but very importantly, the reason you're taking off the top of the log is it isnt flat; its irregular and there will be gaps between the log and the ladder. You cant assume the ladder and log will be in contact all along its length, so you need a bit extra screw length to stretch over the 'gap' where they aren't in contact and might end up with only 10-40mm actually penetrating the log.
I didnt realise "Phillips heads were designed to cause the driver to cam out, to prevent overtightening". Well, they certainly succeed in camming out! I like the look of Torx screws.