So, bear in mind that the tier system is predicated on two main metrics. How well can a character deal with a given problem, and how many different kinds of problem can they solve.
Tier 1 tends to mean that any given build of similar optimization can solve just about any given problem you throw at it or shortly become capable of doing so.
Tier 2 solves a lot of problems, but not as many or not as well as tier one, and no one build can equal the breadth of a tier one class.
Tier 3 is good at one thing in particular, and moderately useful outside of their specialization, or a middling jack of all trades.
Having looked what I think you were referring to (the system presented in true sorcery), it becomes tricky to set it up, because the system relies heavily on homebrew (in effect the ability to create your own spells is one of the core aspects of this system) but the knowledge of spells seems to be severely curtailed. So I'm using the examples given to colour my assessment here.
Spellcasting classes using these rules are immediately barred from Tier 1. One of the defining features of Tier 1 classes is their ability to know all possible spells. This is effectively the bar that keeps Sorcerer in Tier 2, while Wizard is Tier 1. The limits placed on spellcasters using True Sorcery prevent a character from having this ability.
It's also not likely to be Tier 2. Many of the iconic effects that spellcasters produce are better than their equivalent in true sorcery, or simply come online earlier. The force effect is probably the best example of this. A default damaging force effect deals 1d4 nonlethal damage and requires a ranged touch attack, whereas the iconic magic missile would deal 1d4+1 lethal damage with no need to make an attack. It just hits.
So, looking at that, I'd argue a solid tier 3, depending on choices. A True Sorcery spellcaster has tools at their disposal but they seem inherently less potent than a traditional vancian caster, so the result is a lot like a focused casting class, similar to a Dread Necromancer. A True Sorcery caster might even be low 3 high 4 depending on the exact variation used. But given that Thieves' World uses the Mana Threshold so that a spellcaster can arguably cast a more difficult spell over many rounds, it's a little better than default True Sorcery.