I'm not sure what situation you're trying to model. DL works perfectly fine for me. I almost use it exclusively. This probably isn't going to work for you, but I find it REALLY hard to believe you can remove it from your action bar. I don't know what content you're doing that you're so hard pressed for HPS that you're doing nothing but spamming FoL and can't even weave in HS, because I haven't seen it :P
Even in straight spam situation, unless you're around 60% haste, DL will burn less mana per second than FoL spam due to the cast times.
Mana consumed per second @ GCD cap:
FoL 1189 MPS
DL 790 MPS
0 Haste Rating:
FoL 1005 MPS
DL 669 MPS
If you're shooting for efficiency, FoL is not the answer unless your rotation looks like:
FoL -> Stop and do nothing for a few seconds -> FoL -> Stop and Do nothing
I also find it really hard to believe that you've never run into a situation where a tank couldn't use a 20k+ heal, even in WotLK.
If you're running content where HPS isn't an issue, then you should be using HL. And if you're trying to tell me that your HL is overhealing by a lot, then I want to know what content you're running and meet your tank, because now you're in total EZ mode.
EDIT @ Wimplee
Sorry I didn't see your post.
Yes I'm sure about that formula. All haste stacks multiplicatively . You can test this for yourself by using the multiplicative formula and the additive one. Judgements of the Pure and Speed of Light should give you enough variance to see which formula is accurate.
In game, my DL has a cast time of 1.75 seconds w/ JotP, SoL, and 900 Haste Rating (yes I'm a little bit over GCD cap).
Multiplicative formula predicts 1.747 sec cast time. Matches in game data.
Additive formula predicts 1.793 sec cast time.