As it stands right now, I very rarely will go oom in a fight. Generally only if I die and pop will I see an issue. I'm thinking haste will be diminished once I get to 42% unbuffed (1377 rating) but until then do I just keep gearing like I already am? Perhaps gear for more int/mp5 so I can get rid of Soul of the Dead?
Popping back up, not having kings, int, druid, or wisdom buffs is a pretty big chunk of mp5 loss. Not to mention intel which would "weaken" your mana tide. I personally choose to get gear that gives a little more mp5, i think 30%+ + crit is more then I need, but i wish i had that much haste /drool
Its not really had to consider how he'd not go oom, especially on twins when the constant pulse of the auras proc water shield.
Im currently sat on 1014 haste with T9 on, and 977 haste with T8 on(4set to reduce CH cast by 0.2) and i sit with a 1.65 CH cast, never go oom and its not really possible.
In terms of the question, is there such a thing as too much haste? Only if you lack spell power to make heals big enough to count, with 2700SP, you should be getting 9-13k LHW crits, 20 - 25k HW crits, and initial casts of CH being 10k non crit, 15k crit.
If your not going oom, and your healing enough (3500~ SP is enough in a raid enviroment for me doing Anubarak healing Penetrating Cold, and thats the hardest fight in the game atm) just go for more haste, untill you can beat those blasted druids with thier daft instant heals!
With regards to being oom, much depends on the fight and whether it's 25man or 10man. I'm at almost 1k haste with only 20.5k mana unbuffed and I rarely go oom in 25man fights. 10man is a different matter altogether as I'm much more prone to going near oom or oom outright (I'm going to have to re-gear for more regen for 10mans).