I don't think Regain will ever be able to be made useful unless it's set to such a retardedly high amount that it almost sort of takes the place of a pseudo haste. Capped delay reduction, it's not out of the question to be WSing every 3-5 seconds depending on multi attacks. Considering we get tics every 3 seconds, you'd need 100 regain (or 1000, to follow SE's new way of showing it) just to keep that pace up. Stacking the two would be nearly pointless as you can't really WS any faster than that, though depending on the WS, you could benefit from WSing with 200+ TP.
I keep telling myself such a massive amount of regain is broken as hell, but I suppose since a single support job can cap magic haste, maybe another support job that can give massive regain wouldn't be so out of the question either. It'd certainly make Geo much more viable for 6 man fights. Any amount of regain that isn't something so drastic is going to end up being rather pointless though, as it wouldn't be worth using one of your two Geo spells to provide it, except as a pre-fight buff.