As far as I can tell, there are four real options:
- Attribute x1.5
The force 4 Spirit of Fire's strength couldn't be raised higher than 3, this is grounded in the assumption that the spirit's attributes are at their natural maximum. - Force x1.5
The force 4 Spirit of Fire's strength could be raised to 6. This is based on the loose precedent (set in the FAQ) that a possessing spirit's augmented maximums are based on Force rather than just natural attribute. - The greater of the above options
With option 2, a force 1 fire spirit's reaction augmented maximum would be 1, but their attribute would be 4. Making it the higher of the two makes things make a bit more sense. - No Maximum
The maximum strength achievable is based on the force the spell is cast at and the hits generated. This is based on the fact that spirits can be of any force, and thus could potentially reach arbitrarily large strengths, just like how a really powerful magician could reach an arbitrarily large magic attribute. The spirits generally summoned by PCs may have fixed forces, but there are other spirits that have variable forces, and any free spirit can increase their force.
For balance's sake, I'm leaning towards 1, but that really is a pitifully small boost in some cases. It also lets you reach really high numbers when spirits have bonuses already, letting you boost an equivalent Spirit of Earth's strength to 12.
Also, am I the only person who's realised that possessing spirits can have their mental attributes pumped a lot more than you might expect according to the FAQ ruling on maximums? Lets say a Honguan force 4 spirit possesses a character with Charisma 4. According to the formula, the augmented maximum for Charisma becomes 12. Sure, it'd take a force 8 Increase Attribute (Charisma) spell with 8 hits to raise it from 4 to 12, but it's not impossible thanks to the low drain of the Increase spells. In-fact the drain for the force 4 spirit is likely to be higher than the drain for the force 8 spell, although the spell drain would probably be physical as you'd be overcasting.
Lets say that spirit had the Influence power (Spirit of Man, or a Guardian spirit with the optional power) ... It'd be rolling 16 dice, the target would just be rolling Willpower.