Ok, I'm home now. Time for my own sanity check of the math:
Marginally, 1 Spi = 1/8 MPS. Since I'm always inside the 5SR, with my 2/3 Med, 1 Spi = 1/8*0.4 = 0.05 MPS. However, since I'm a Human with Kings, it's really 0.05 * 1.15 = 0.0575.
1 MP5 = 0.2 MPS.
So, 11 MP5 will equal 20 Spi on a fight of length T seconds, where 0.2*T*11 = (16 + 0.0575*T)*20 => T = 305 seconds.
Okay, close, but not quite. The difference is that I tried to account for the full Spi tick that sometimes sneaks in when you Clearcast a Fireball. Given that with some casting lag, each Clearcast can lead to over 1.5s out of the 5SR, it's noticable.
Anyway, being more exact:
On a 420 second fight with 1 Evocation, 10MP5 is as valuable to me as between 19 and 20 Spi.
On a 300 second fight with 1 Evocation, 10MP5 is as valuable to me as between 16 and 17 Spi.
Yup, nothing to re-enchant your bracers over, but unless someone points out a gaping hole, that's the equivalence.