Before I used grid quite a lot and didn't find any problems with tunnelling than I would any other raid frame. what I liked with grid is that is showed (with numbers) when someone was getting healed so you don't over heal someone as opposed to the off colour bar that default raid frames have when healing someone.
After using quite a lot of addons and numerous tweaks I have since started to use only the ones I absolutely cannot live without.
PVP control: I use this addon for two reasons, 1st being that when you are cc'd/countered (fear, stun, poly, etc) it will alert everyone around you that you are cc'd or interrupted in /s or /p. What it says depends on what you make it say. 2nd being that it gives an emote when you have low mana. While vent makes many of these things moot, when you are pugging or even semi premading it helps out those around you that aren't in vent or if your like me and too lazy to get in vent most of the time.
Capping: obvious reasons
Battleground targets: Helps you with your gear/strat since, as a support class, your gear should be a counter to what the opposing team has allowing you to best help out your team.
Deadly Boss Mods (DBM) : Used to track wsg debuff.
GladiatorLOSsa: tells you through voice when cooldowns are used. Very useful!
Spell alerter: Helpful tool to track enemy cooldowns, especially because it shows the target! I use it mainly to know when my team mates have been interrupted/cc'd and to help with shadowstep --> kick.
Vialcooldowns/icicle: Used to track enemy cooldowns. I prefer GladiatorLOSsa + vialcooldowns over interrupt bar because with interrupt bar it only tracks it once, so if there are mutiple rogues/mages that you are trying to juke you are in for a VERY rough time.
The next few are just ones that I really like, but you can still play effectively without them.
Simple Holy power: For a Hpally I cannot live without this. It gives you a bar that you can place anywhere on the screen that shows your holy power. While simple, you'd be surprised how much it makes things easier.
Omni CC: shows numerically the cooldown remaining overtop of everything. Very useful!
Bagnon: Being that I'm a gear whore with multiple sets, having one giant bag not only helps with organization, but also with changing gear quickly and easily.
Quartz: While being very similar to the default castbar, what makes this useful is that it shows your latency and gives you a tiny red bar at the end of the cast bar and if you stop right in the red portion of the bar that cast still completes. This "red portion" is your latency which other cast bars do not show. You still can do this trick without quartz, with quartz it is MUCH easier and while this only makes your casts a TINY tiny bit faster, why not give yourself a slight edge over everyone else?
While all of these addons do not directly impact your healing like grid, vuhdu, or any other raid frame addon or even clique might, what they do do is provide you with tools to help you be the best support class that you can be.