So here is a question for the assembly.
Spell Mastery. I have seen it used to multiply hits delivered by x2, x3, x4 and even x5 (by a PC, accumulative -20 per multiple). I personally require -30 since multiplying hits strikes me as similar to +1 target. Anyway, should Spell Mastery be able to increase the % of hits lost? -20% could become -40%(-30), -60%(-60), -80%(-90), -100%(-120), etc. Imagine -50% becoming -100%(-30), -150%(-60), -200%(-90), etc.
Two ranks per level in Spell Mastery seems unlikely unless the skill is EV (via a talent perhaps). But it is possible, so, at two ranks a level, at level 15 our Sorcerer/Mentalist could have 30 ranks (+95). With exceptional stats and such (+40), resulting in a skill of 135. At -90, our spell user better roll well, but at -60, an average roll will result in success.
At one rank per level, it takes till level 30 to reach this skill level, all other considerations (bonus items, talents, etc) aside.