As a marginal fan of tournament golf, I'd be pretty disappointed if a tournament were decided by a penalty stroke given for slow play. Not that it would be inherently unreasonable- I could see where it would be entirely appropriate- but just that it would leave a general bad taste to watch a tournament and have it decided that way.
So how about this, instead of stroke penalties, the Tour goes right after what counts, the wallet.
Assume a baseline of 4:30 for each round- 270 minutes. If you finish slower than that, what ever percentage of time you were late, gets deducted from your winnings. So if you finish in 4:57, you're 27 minutes late and you just donated 10% of your winnings to the Make-A-Wish foundation (or something similar.) This way the result of the event isn't compromised, but the point is clearly made.
You can adjust the 4:30 depending on course and conditions, and you might even be able to apportion fault within the slow group if necessary, though I'm not quite sure how that would work practically. Of course, you might need a lot of security to keep a few guys from getting killed when this rolls out, but I think I would prefer this option to penalties. Any chance the players would go for it?