Effective mp/5 from spirit?

Ok, so a while back, I posted about which was better, intellect or spirit. I eventually concluded that spirit was best for healers but not for dps. DPS generally stay in the FSR because they're casting as often as they can. While healers only need to cast when someone's hurt, they can benifit from spirit-based mana regen.

Now, with that said, which is better, spirit or mp/5?

Instead of comparing the mathematical forumlas, wowwiki describes the relationship between spirit and mp/5 in terms of enchants.

"For example the two top mana regeneration enchants for chest items yield either 15 spirit or 6 mp5 ([Enchant Chest - Major Spirit] and [Enchant Chest - Restore Mana Prime]) - this shows the factor of 2.5 very clearly."

They surmise with this, that 1 mp/5 = 2.5 spirit. I don't agree. I think that mp/5 is worth far more spirit. To demonstrate, I've done some calculations using excel again with the same input parameters as I did in the intellect vs spirit discussion. Assuming a character has 1000 intellect and 500 spirit, the following graph shows their equivalent mp/5, depending on their time spent outside the FSR.

Realistically, the value of 1 mp/5 compared to spirit depends largely upon time spent outside the FSR. Let's assume that a priest spends 25% of their time outside the FSR (which is fairly generous, since we decided that 13% was the break even point last time). At 25% outside the FSR, the effective mp/5 from spirit is 79. Now, if we divide the spirit required by the acquired mana regeneration, you get:

500/79 = 6.33 spirit = 1 mp/5

mp/5 is worth 6 times more than spirit, not the 2.5 that wowwiki suggests.

Now, say you think you're an exceptional healer, and get 50% outside the FSR:

500/132 = 3.79 spirit = 1 mp/5. That's still higher than wowwiki's 2.5 suggestion.

In fact, if we solve the equation 500/x = 2.5 (x = 200), and compare that mp/5 value to our graph, we find that we'd need to spend 75% of the time outside the FSR to achieve this mp5 to spirit ratio.

So, spirit is disproportionally availiable on gear. Take gems for example. The highest epic gem granting spirit give +20 spirit. While a +10 mp/5 gem is availiable. The mp/5 is worth +63 spirit according to our calculations, and doesn't have the FSR limitation.

I've now geared my priest so that it has 150 regular mp/5, with 965 spirit and 1182 intellect unbuffed which equates to 139 mp/5 at 25% FSR. A total of 289 effective mp/5. I'd love to swap out all my spirit for mp/5 if I could. At an exchange rate of 6.33 spirit per mp/5, I'd get 152 additional regular mp/5. The problem is that it's just not availiable in those quantities. Since gear is lacking in mp/5 equip bonuses, it is impossible to swap out all your spirit for mp/5. I could trade out some gems and upgrade a few pieces, but it's tough to come across, especially below raid-quality gear. Healing gear comes pre-loaded with spirit, just not at the exchange rate we'd expect. I assume this is to keep the difficulty level of the regular and heroic 5-mans.