Your personal attacks don't exactly endear you to me but I want to try to help here because you seem like a new healer and I have a soft spot for new healers. Please read what I say with an open mind?
I've just really gotten into doing Savage this tier, not that I've cleared o8s or anything like Miste, Cass and Hyo but I think my perspective is unique from theirs in that I haven't had Savage as a big part of my gameplay for very long.
I play on ps4 so I don't parse but I've had friends parse me for quite a while, I've done respectable dps as a healer in all the content that I've done. I hit Savage and went from collecting purples and oranges (75th to 99th percentile) in Rabanastre to barely being able to fight my way out of grey (so under 25%) and it was immediately discouraging. I, like you, severly underestimated the chasm of a difficulty gap between dungeons and Savage. This is my first suggestion to you - speak on what you know not what you think may be true. If you haven't completed a piece of content don't assume you know what it's like because you've done other content. Going from dungeons to Savage is very much like playing a different game entirely.
Next lets try to address the downsides of overhealing. You've said you haven't experienced any mp issues, that's good! Managing your mp is important and it will serve you well in the future. Another issue with overhealing though is enmity. As an AST you have a built in enmity reduction (which I am jealous of on WHM) but at a certain point you will still start forcing your tank to adjust to keep hate off of you. The only skill you have that sheds enmity is Lucid Dreaming which (in most content) you want to use for mp recovery more than the threat dump.
This is where overhealing starts becoming actually bad - content may be tuned without healer dps in mind but it is tuned with tank dps. You're going to be costing your tank dps because he's going to have to use more enmity combos to hold off of you. Now you're not just negatively impacting your own mp but also someone else's dps.
Will that matter in a dungeon? No, but you're talking about walking into Savage and I want to help save you some headaches.
Finally we come to optimization. This is (as usual) the thing that everyone else focused on already. Your personal dps as an AST is nothing to laugh at, especially in a situation where you can AoE. Gravity + Lightspeed is an amazing combo now. Even in single target situations if you are filling your downtime with Malefic III and keeping Combust rolling you aren't going to sustain the same level of MP that you are now and, objectively, it's better to spend the mp and GCDs on some damage than on literally nothing because that's what overheal is - wasted healing. It's like damaging something that's already dead, kinda pointless right?
I hope you can learn from this thread and from branching out and doing some new content. Byakko EX is pretty easy at this point since people know the mechs, give healing that a go maybe. I also hope you understand that this comes from a place of trying to save you the frustrations I know will lie ahead of you if you continue as you are but try to move into more challenging content. From the arrogance to the overhealing it will all make it very difficult.
Hope it helps.