20% of a base or 8% of a total is just the difference in how it stacks with other effects. Cheaper costs, faster casting, more no save spells (edit: referring to damage spells vs CC or instant death), lack of spell penetration checks on damage spells, and savants while looking at a high base still makes it a decent capstone. How it should stack would require a dev comment unless someone has a quote to link.
On your edit, given just the capstone and no PREs then a wizard capstone does nothing but give 1 more DC and a bit more leeway with SP that still would not match sorc SP. Give them both 1 more DC and then keep the damage bonus and there is a lot less reason to consider a wizard. Right now sorcs are in a good spot as damage casters.
Acolyte of the Skin I am not entirely sure how it will be implemented. Even without using it as a DC PRE I would be more inclined to suggest a DC based PRE for sorc instead of a capstone change.
I do not under stand why a sorc should get a DC 'at least as high as wizards'. The wizard advantage is a higher DC, more feats, and more spells slots with easier swapping. The main advantages are the feats and higher DC. Sorcs still have caster level bonuses, cheaper damage spells, more damage, shorter cooldowns, and a bigger SP pool. Giving a sorc equivalent or higher DCs just moves the preferred caster back to sorc over wizzie again. I think the balance (edit: between sorc and wizzie) is nice now with the spell pass.
If there were to be a suggestion to add a DC PRE for sorcs who want to focus that way I would be more inclined to sign that as opposed to giving more benefits to savants. To me that makes much more sense, and I am smart just ask me.