You want to atleast shoot for 1860 haste to get that 14th tick of Holy Radience. After that I would suggest more into Mastery for IH. Atleast for 10 mans. Now in 25 mans some Holy Pallys are going for that Mastery build.
wow. my haste is 980 which according to everywhere else is shite loli take it i should regem for Haste/Int then and put mastery on the back
Gemming for anything except for Intellect (Red, Orange, or Purple Intellect gems) is a bad idea, no matter what your current Haste levels are. If you want more Haste, get it by reforging Crit/Mastery for it, or maybe use your Alch trinket instead of the Jar.
I'm not 100% sold on the "Haste is always king in 10ms" concept either, I've been testing out Mastery and I can see it working with some raid comps. Personally, I have a very skilled Druid and Shaman who have my back, so I can focus more on the tanks (obviously, this doesn't mean TUNNELING on the tanks, but making them your highest priority).
Of course, I'm not the most progressed raider in the world at the moment (4/7 atm) so what works for me may not work in the later fights. We'll see.
In my 10 man I am usually healing MT and BoL OT so with a Mastery of 11 post 4.2 my IH increased by 7% which is nice. However with my haste at 1863, that extra 14th tick from HR really helps out the raid healers. Just shoot for what works for your playing style. If people are staying alive and your clearing content then your probably good where your at.