10 spell speed gives you exactly a .01 drop in cast speed, so 24 spell speed is a .02 drop (spell speed rounds down to the nearest 10). Also, weapon damage for heals is very, very significant. It serves as a multiplier to all your other stats, and a 30-40 jump in heals from a new weapon is not unimaginable.

As far as Det goes, I don't think anyone's done the tests for healing, but we do know that its value varies pretty wildly for different dps classes.

From that being said, of what I've done in coil, I've always felt like most of the healing is quickly filling up the tank after they get chunked (think Titan phase 3, constantly). In that regard, being able to put out other heals quicker (or being able to debuff and have your GCD come back quicker) feel more important than marginal heal-per-cast increases, but the difference is still negligible.

Of course, you could just make ilvl 70 crafted items and load them all with det, spell speed and crit!