Thanks for the feedback. You raise valid points and I'll try to answer some of them.

I see that people consider the capstone very powerful.
It is, that is without question, but I don't really see how it would be too powerful.
A level 20 circuit mage who never took Additional Circuit would only have 20 circuits.
If he were to now concentrate only on level 9 spells (which he now can use due to compression), he could still only have 3 of them at his fingertips at any time.
Granted, 3 level 9 spells at-will can be devastating, but its also somewhat predictable. I guess it could be broken easily by Circuit Blueprints.
Though I think if I were to double the cost it would get better. (A 6 circuit blueprint would then cost 36,000gp to make).
The restriction that the spells need SR:Yes and a target at least reduces summon monster or disjunction shenanigans.
Circuit Compression was actually the secondary choice for the capstone. The original one was to allow the circuit mage a final mastery over metamagic by only having the base spell need to confirm to the maximum circuit limit, not metamagic circuits. So a circuit mage could have a repertoire of a few low level spells that she could strenghten by however many circuits of metamagic she wants/has.

The spell resistance was a wilfull deviation from the warlock. Originally I had envisioned a scaling resistance:all for those steps. The fluff is that the circuit mage learns to weed out the things her own circuits can recognize as magic. She has managed to open/close her being to wanted and unwanted effects to an extend. Its also not that strong. If the opponent is of the same level then for a long time the resitance is only 25%.
I guess the fact that SR isn't common as a class ability makes it a powerful choice. Generally a circuit mage won't want to actually have the SR up as its more beneficial (at least once you have the ability) to gain rapid immunity for whatever spell is thrown at you.