Not entirely correct. Conserve MP procs 25% of the time with no +conserve MP gear or weapons. The amount conserved is also random, ranging from 1/16 to 8/16 of the total amount of MP of the spell used. That's to say, if you use Haste or refresh, it would save anywhere from a minimum of 2(or is it 3 because of rounding?) MP to a maximum of 20 MP.
The spell cost is rounded down, so it's basically the amount saved being rounded up.
40 MP - no CMP
37 MP - 1/16 CMP
35 MP - 2/16 CMP
32 MP - 3/16 CMP
30 MP - 4/16 CMP
27 MP - 5/16 CMP
25 MP - 6/16 CMP
22 MP - 7/16 CMP
20 MP - 8/16 CMP
Conserve MP gear is irrelevant to the discussion: it'll give you just as much extra MP savings with either sub, since it's simply a linear increase to proc rate. I can do the math out if anyone doesn't believe me.
While it is a linear increase, the proportional amount saved varies, although only slightly. It's like haste, while you end up only saving 2% of your base MP spent, with CMP that's a 2.2% savings of your actual MP spent, just like adding 2% haste to 7%.
The point where ballad and refresh come in is that they give you extra MP to use, so you're not relying on your resting to get it all back.
While you would need about 2800 base MP spent in 10 minutes to make up for the MP of /smn, you will get almost 200 of that back from CMP procs, meaning your actual MP spent will only be 2600.
With double ballad, refresh, and noble's/relic body you're getting back 1400 MP over 10 minutes, so you're down to 1200.
With Choco cake (hMP+6, 3 hours and cheap) and a good hMP set you'll be getting 57 MP first tick, then 59, 61, etc (the autorefresh on full yigit set is cancelled out by the loss of relic/nobles body to equip it). That means it would take about 4 ticks of healing every 2 minutes to make up the difference, which is 45 seconds of resting.
Recovering enough MP to make up the difference isn't that hard, especially if you have backup healing. The hard part is spending enough MP for it to actually reach that point.
As for whm casting sleep, they have about an equal skill level to blm when geared up and blm doesn't have trouble sleeping most MP mobs with merits and enfeebling gear.