FFS is a great spell, but it's my impression that a longer duration (or a lower mana consumption) isn't worth extra skill points if you compare it to the proposed effect increase of a fireball. An increase in the quality of the spell might be necessary, so at level 1 people might have a certain chance of still noticing you because they can hear you or your veil is imperfect. Whether additional effects for Wizard's Sight would be worth more skill points is hard to tell. My point was that if Larian finds it hard to balance spells and to increase certain effects, it would be better to have lower maximum levels for some spells than to have extra levels that aren't really worth the skill points.

For the quality of Invisibility to increase, creatures close to you can see your blurred outline. You still gain a hefty defense boost, but can be targetted and attacked. The range creatures can see you within decreases each level. You're never be completely invisible, but medium-to-high levels will stop pesky archers and wizards from hitting you from afar.

I get your point with lowering skill maximums. Each skill level that can be bought must be of equivilent value to other skills. Don't bother offering moot skill levels just because "every skill should have x levels."