Well, I finally did it. After the good times we had, I decided to let my FoL spec go. It was a lot of fun, and I still think it's a viable spec in some situations. (Un)Fortunately we're working on 4/12 H ICC25 and 9/12 H ICC10 - and it just can't keep up. Especially when we're pulling healers out to DPS. The raw throughput of Holy Light is just pushing past what I could dump out with FoLs.
Haste is a factor
I'm also at a point now where I'm pushing the 1.2 second Holy Light barrier. With gear and a Manta Ray, I'm at 832 haste when the fight starts. Even though my mp5 (and possibly crit) are a little lower than I'd like, I can still pump out the HL for a good while. It also doesn't hurt that I can usually count on one or two innervates throughout the fight. While I can certainly manage my mana without them, it prevents me from juggling the Divine Plea debuff. Using the 2pc T10 and Avenging Wrath cooldowns certainly help, but it's still not 100%.
Of course there are times when I have to call for an innervate because I've gotten tunnel visioned - but that's something I can fix. More mp5 can't hurt, and I've got some pieces I can start swapping out now that I'm over the base 680 haste cap.
The real reason
The real reason that the FoL build died though, was that I just can't afford to be without the buffs that 51/20/0 provides - and that spec doesn't provide enough crit (imo) to make a FoL build viable. Losing out on Imp Devo aura (in 10s) hurts as well, and DS/DG is still an amazing talent. Unfortunately, there's just not the HPS I need, and there's no point in having a 2nd Holy build for FOL, if I'm never going to be able to use it.
So it's time to spec Holy PvP and annoy people.