[Phanx]

I'd go with something like this:

local healBuffs = {
	[(GetSpellInfo(47788))] = 1.6,  -- Guardian Spirit
	[(GetSpellInfo(55233))] = 1.25, -- Vampiric Blood; needs to be adjusted for glyph
}

local healDebuffs = {
	[(GetSpellInfo(8680))] = 0.75, -- Wound Poison
}

local unitIncomingHeals = {}
local unitModifiedHeals = {}

local MyAddon = CreateFrame("Frame")
MyAddon:SetScript("OnEvent", function(self, event, ...) return self[event] and self[event](self, ...) end)
MyAddon:RegisterEvent("UNIT_INCOMING_HEALS")

function MyAddon:UNIT_AURA(unit)
	local amount = unitIncomingHeals[unit]
	if not amount then return end

	for buff, mult in pairs(healBuffs) do
		local exists, _, _, count = UnitBuff(unit, buff)
		if exists then
			amount = amount * (mult * (count or 1))
		end
	end
	for debuff, mult in pairs(healDebuffs) do
		local exists, _, _, count = UnitDebuff(unit, buff)
		if exists then
			amount = amount * (mult * (count or 1))
		end
	end

	if unitModifiedHeals == amount then return end
	unitModifiedHeals[unit] = amount
	
	print("New heal amount on", unit, "is", floor(amount))
end

function MyAddon:UNIT_HEAL_PREDICTION(unit)
	local amount = UnitGetIncomingHeals(unit)
	if amount and amount > 0 then
		if not next(unitIncomingHeals) then
			self:RegisterEvent("UNIT_AURA")
		end
		unitIncomingHeals[unit] = amount
		return self:UNIT_AURA(unit)
	else
		unitIncomingHeals[unit] = nil
		if not next(unitIncomingHeals) then
			self:UnregisterEvent("UNIT_AURA")
		end
	end
end

Also, if you want a spreadsheet of all currently known healing debuffs, including spellIDs, amounts, stacks, sources, etc. send me a PM with your email address.