Yeah, it sounds like you're getting a good deal of utility to augment your spellcasting. Sounds pretty close to how it works in the novel too.
How do you think 4 refresh of soulfire stacks up to 4 refresh of mental toughness (2 refinement and inhuman mental toughness)?
I dislike Mental Toughness, so my house rule is it doesn't apply to self-inflicted mental stress (so, spellcasting). By extension, the Refinement requirements are eliminated and anyone with a good reason can take it (which I call tiered "Will" rather than throwing "Mental" in front of "Toughness"). (Demonic Co-Pilot is
not self-inflicted stress, technically.)
Since I haven't seen it in action vs Soulfire, I can't say for sure, but I would venture to guess that the IMT is better. Talking strictly evocation optimization, since these abilities are less useful for Thaumaturgy, assuming a Conviction and Discipline of Superb, zeroes on Control rolls, and four focus item slots spent on a +2 Offensive/Defensive Control item, the math is below.
Inhuman Mental Toughness:+2 boxes of stress and Armor: 1 against mental stress; I believe the general consensus is that the Armor doesn't apply to spellcasting so I'll assume that. So, you get the typical complement of spell options, plus two more potential spells per scene, one at a potential +4 and one at +5 power, too.
-Your apex element can be cast @ +2 P/+1 C (assuming you have no more than the two Refinement)
- With this in mind, you can cast the following spells per turn, assuming you're using your apex element and are not worried about absorbing Backlash, tagging stuff, or spending FPs for enough Control:
*7P/8C
*8P/8C
*9P/8C
*10P/8C
*11P/8C
*12P/8C
((105 shifts!))
Non-Apex Elements: (Note, you still have +2 more Specialization points floating around from your refinement, so I'll assume your secondary element is +1P/+1C)
*6P/6C
*7P/6C
*8P/6C
*9P/6C
*10P/6C
*11P/6C
((87 shifts))
Soulfire:
+1, +2, and +3 power on one spell (of any kind) each.
- Assume +1 to Control apex element w/ Evocation Specialization
- 3 spells that downgrade Toughness by 1 level (== free +3 Stress on successful strikes w/ attacks vs Tough opponents, effectively, given that most Attack spells strike in the upper range of an opponent's stress track)
- Possible +1 add'l point of power on each of those spells based on narration (easy to do), so now we're at +2, +3, and +4 Power
- With this in mind, you can cast the following spells per turn, assuming you're not worried about absorbing Backlash, tagging stuff, or spending FPs for enough Control:
* 9P/8C (+3 stress on successful strike)
* 9P/8C (+3 stress on successful strike)
* 9P/8C (+3 stress on successful strike)
* 8P/8C
((67 shifts in total. There are different ways of applying these bonuses of course, but the total number of shifts remains the same. The biggest possible spell you could get off here is +12P/8C if you combined your 4th mental stress box and 3rd Soulfire stress))
Non-Apex Element:
*5P/5C
*8P/5C (+3 stress on successful strike)
*10P/5C (+3 stress on successful strike)
*12C/5C (+3 stress on successful strike)
((64 shifts inc. assuming all spells strike))
My Assessment:With apex-element casting, you can pull out 57% more shifts with IMT vs SF. With non-apex-element casting, you're getting 36% more shifts w/ IMT vs SF. If all your Soulfire spells are not Attacks, this ratio gets worse.
I don't consider the Holy catch especially worth anything much, since Fire in particular can cover a lot of thing's Catches too. There are ways of statting these options more optimally but I wanted more of an apples to apples comparison. On top of the extra shifts, getting to cast 2 additional spells per conflict, to me, is seriously powerful and tips the scales to me (which is why I don't like using it in the first place).
EDIT: Soulfire grants additional flexibility with narrative freedom from Evothaum, in addition to the ability to lay out several aspects at a time with evothaum maneuvering, which is powerful--but you only get a few of those and it's not as useful for outright offense vs an Attack spell. The flexibility is worth something, but not a score or more shifts per conflict, I'd say. Your spells appear a little easier to balance with Soulfire in terms of power vs control, but in practice I don't think that's as important a consideration. Soulfire is a pretty solid addition to magic--the PP RAW IMT is
broken powerful.
EDIT II: Fixed some math.