The RPPM trinkets do get a bit more uptime from higher haste, but chances are you'll still favor mastery and spirit more since even with the extra trinket uptime they're going to be better stats usually. Depends on your healing style.

If by extra tick you mean another tick on Sacred Shield, then the spreadsheet I made here can answer that. Sacred Shield's extra ticks come every 20% haste after the first, which is 10%. Eternal Flame, however, is every 10% after the first tick which is at 5%. So it can be 25%, and the haste required for that assuming SoI and the 5% raid buff would be 3496, maybe going up to 3500ish due to rounding.

The 25% haste breakpoint is actually fairly decent even without factoring in EF. It gets you to 2 seconds flat on DL and HL and 1.2 seconds on FoL and the spell GCD.

Breaking T14 4p is definitely going to hurt, and that goes for everyone. If you're on AoE healing duty though, using 2p T14 and 2p T15 might work pretty well instead of 4p T14 since the 50% extra healing from Daybreak is larger than the 33% reduction to HS's CD.

It might be worth it to break 4p T14 and got with off set pieces and T15, it might not be. When you do get the pieces, try it out and see which works best for you and your raid. That's about the best that can be done really since healing is such an odd beast. Again, that'll go for pretty much everyone.

Correct, EF's extra ticks come at 5%, then every 10% after so at 15%, 25%, etc. If you ever go for SS instead, those numbers are there too.

The spreadsheet I made accounts for buffs though, and to reach the 15% all you'll need is Seal of Insight active with the raid getting the Spell Haste 5% buff. That already puts you past 15% since the haste effects are multiplicative. Going up to 25% though would be about 3500 haste assuming you have SoI and the raid buff up.

Mastery plateaus will only happen if you're doing easy content or get a massive healing buff. The easy content of course would require no stat redo because it's probably not worth it. The massive healing buff fight might be worth trading some items around for, but if your only option is reforging then you probably shouldn't bother unless it's a progression fight on the cutting edge and you're willing to spend that much gold to eke out a few more percent.

The short duration and spacey proc do seem, at first glance, to hinder it a bit compared to the Revitalizing. However, if we do assume it procs 1.4 times per minute (it's 1.4 RPPM, so possible with their new proc fixes to prevent long times without a proc) then it'll be equivalent to 5.6 seconds per minute. Assuming we cast 3 FoL in that time (possible with a HoPo spell mixed in), then that's 68040 mana saved that minute, or effectively 5670 Mp5. If it's 2 DL during that time then it's 43200 mana saved, or 3600 effectively Mp5.

If it's simply used on 2 HL though, it's 7560 mana saved, or 630 Mp5. A single DL would save 21600 mana for 1800 Mp5.

If it manages to proc even once a minute, it should be comparable to or even beat the Revitalizing in terms of mana savings (and the Int on it handily beats the crit bonus from Revitalizing). The issue is actually noticing the proc go up to make use of it via DL, FoL, or HR, but that can be done via an aura addon.

Can you test to see whether it procs on EF or SS ticks?

Points of view may differ.

I actually see it as a nerf to daybreak (only slightly compensated by adding more versatility), and a nerf to our overall absorbs resulting in a small increase of the value of mastery rating.

Regarding daybreak, the big change for me is that per HR cast you only increase the next holy shock by 75% (while it is 100% currently on live). Of course in the new version, you can chain cast HR more and still benefit from the daybreak gain for 2 casts between each HS instead of 1. I am not sure we will have the mana to do that consistently, but it might prove useful now that the CD on HS is back to 6 sec.

Regarding our absorbs (illuminated healing), if I read the notes correctly, they only reduce the fixed part which does not depend on mastery rating. This is just a flat nerf to our overall healing, but the part which is due to mastery rating should remain the same and thus contribute proportionally more to our overall healing than what it does now.