You might be on to something there, Sadrik. Let's stick with 1d8, since that's what divine spells use, but use the base amount + 1d8/2 levels, +1/caster level, with the standard cap (CLW at L5, CMW at L10, CSW at L15, and CCW at L20. So, with CLW, we get...
L1: 1d8+1 (same as now)
L3: 2d8+3 (vs. 1d8+3)
L5: 2d8+5 (vs. 1d8+5)
CMW becomes:
L3: 1d8+1d8+3 (same as now)
L6: 1d8+3d8+6
L8: 1d8+4d8+8
L10: 1d8+5d8+10 (37; not very much at L10, but still useful)
CSW:
L5: 3d8+2d8+5
L8: 3d8+4d8+8
L10: 3d8+5d8+10 (46; better than CMW, but not overwhelmingly so)
L15: 3d8+7d8+15 (60)
CCW:
L7: 3d8 + 3d8 + 7 (better than 4d8+7, but less than CSW at L8 and more than CLW at L8, which is good).
L10: 3d8 + 5d8 + 10
L15: 4d8+7d8+15 (4.5 better than CSW at the same level, which is comparable to the existing spells).
L20: 3d8 + 10d8 + 20 (78. A respectable number).
Compare to heal: 10 points/level, max of 150 at L15.
The mass spells (vanilla) work this way: Extra healing power is capped at 5 x CL. Why they did that is beyond me, but whatever. Since mass spells always have a lower number of dice, I would probably (off the top of my head) go with base amount + 1d8/3 levels + 1/level, with the cap at 3 x CL. So...
Mass CLW (caps at L15)
L9: 1d8 + 3d8 + 9
L12: 1d8 + 4d8 + 12 (34)
L15: 1d8 + 5d8 + 15 (42)
Mass CMW (cap at L20)
L11: 2d8 + 3d8 + 11
L14: 2d8 + 4d8 + 14
L20: 2d8 + 6d8 + 20 (56)
Mass CSW (caps at L25)
L13: 3d8 + 4d8 + 13
L20: 3d8 + 6d8 + 20 (60)
L25: 3d8 + 8d8 + 25 (74)
Mass CCW (caps at L30)
L15: 4d8 + 5d8 + 15
L20: 4d8 + 6d8 + 20 (65)
L25: 4d8 + 8d8 + 25 (79)
L30: 4d8 + 10d8 + 30 (93)
My conclusion: These are very workable numbers. They're hardly game-breaking; at L20, your average mage will have (2.5 x 20) + 20 = 65 hp, assuming a +1 Con bonus. The average rogue/monk would have around (3.5 x 20) + 40 = 110 hp; a cleric/druid would have around (4.5 x 20) + 60 = 140, and a fighter-type would likely have around (5.5 x 20) + 100 = 210.
These are probably lowball figures; I had a rogue multiclass with 440 hit points at L30, and the barbarian/Legendary Dreadnaught had twice that, but the point is that these changes make healing much more useful without going as far as 4E's "heal completely between battles" thing. Clerics can carry around fewer healing spells (or sac fewer spells for healing), while druids and bards become more efficient healers, and potions/wands are more effective. Basically, it's a win-win situation all around.
And on the other side of the coin, using cure/inflict spells against enemies becomes a lot more viable as well. Hit a vampire with an empowered CMW (or god forbid, a maximized one), and he won't shrug it off like he does now.