Yes, the higher level heals cast faster. The Phoenix character builder shows the cast speed for each spell.
As far as decreased power goes they are very power efficient, that being said a dedicated healer has a better variety of spells for this job so you will never replace an actual heal class.
I ran 35/24/33 for a while as a jack-of-all-trades build, and while the dps was pretty crappy it gave me a lot of tools to play with (including 400+ heals) and I had a lot of fun with it.