"Obviously 22ga at 40 feet is a bit silly but so is using 10 or 8ga wire."
If I could find some good, reasonably priced in-wall 8ga wire I would most definitely install it. Who knows what future upgrades may be, and who knows what the impedences of those speakers may be. Just looking at the length of run and the wire size works in most cases, but a speaker that is rated as 4 ohms may actually dip into the 2 ohm range at certain bass frequencies, and such a speaker connected to an amp by a 40' cable would definitely perform better with 10ga wire than 12ga wire.
