With my current haste raid buffed, without BT, BL, or PI my Flash heal cast time is 1.02 seconds. My Greater Heal is 1.67 seconds. Take your FH/GH crit numbers and apply it to those cast times. That's 9,000/1.02 = 8823, or 17,000/1.67 = 10,179.
With BT? That's a 1 second FH due to GCD cap, or a 1.35 second GH. With BL? 1 second FH due to GCD cap, 1.28 second GH. With BT you're looking at just under 12,600 HPS and comparing it to 9,000 HPS and claiming that FH is higher throughput.
Why are you talking about sniping? You're not a Holy Priest. Heck, you're making fun of Greater Heal's coefficient? It's 161.35%, Flash Heal is 80.68%. If you quantify that by cast time you'll find that Flash Heal scales about 10% worse than Greater Heal.
You have no idea what you're talking about and have no clue why this is working for people.
Here's the issue: Disc lacks throughput, but has a massive advantage for mana gains.
Your answer: Spam low throughput, low cost heals.
The actual solution: Use higher cost, higher throughput spells.
You also don't seem to understand the haste levels involved. Priests in high end gear are now approaching the GCD cap without BT. With BT Flash Heal is just about .8 seconds. Not that I can cast it that fast, I'm well beyong the GCD cap with BT. Know what spell that doesn't matter for? Greater Heal. Side effect? 1.6 second Prayer of Healing. Secondary side effect? Gives a great third option for Glyphs. Tertiary side effect? My maximum output is 30-40% higher.