I agree with Bodhi about the leveling time - I've been switching between holy and ret, and holy is about an hour more per level, which is still a lot better than it was in the old days. The more quest-intensive the area the quicker it goes, of course.
I'll hit 75 tonight, and I haven't replaced any of the main (weapon/armor) pieces of my T4/badge set, just necklace/cloak/rings, and those were primarily instance drops and instance quest rewards. I'm starting to see some world quest rewards late in Dragonblight that make me think, but I don't expect to replace any core pieces until Zul'Drak.
As for healing offspec, it's certainly possible and Blizz has made it a goal that it should continue to be during leveling, but I'll still respec (for the practice if nothing else - healing as Ret feels like 2.4, and my style is different now). I know that the final boss in Nexus would have been a nightmare without the HS/Infusion combo.