I think it doesn't make sense, thematically and semantically, to do what y'all are saying. For example, take a level 1 Elf spellburning the hell out of a patron bond for obvious reasons.
The way y'all have suggested it goes, a guy who ROLLs a 20 and spellburns 1 point will be doing much better than a guy who spellburns 20 points and doesn't roll; likewise a guy who rolls a 10 but spellburns 18 will be doing wayyyyy better than the guy who didn't roll. Considering what spellburn really is... how does that make sense?
I also don't really agree with the RAW above... I think you should have to roll every time, no matter what. That way, there's always the chance of an abyssmal failure/corruption. I wouldn't allow my players to take a 20 without rolling. But, frankly, if you're spellburning 20 stat points, that's a major, major undertaking.
Hamun Ry (Wiz 4)
Str 10 Agi 15 (+1) Sta 11 Per 11 Int 17 (+2) Luc 10 (Unholy House). Align: C. AC: 14. HP: 13. Melee +1, Ranged +2. Crit: d8, I. Save: Ref +5, Fort +3, Will +4.
Spells: 1: Choking Cloud, Comprehend Languages, Detect Magic (odd crystal growths), Magic Missile (mirror images), Runic Alphabet (Mortal) (ravenously hungry), Ventriloquism (rain of frogs)
2: Detect Invisible, Levitate (extremely difficult, d14), Mirror Image (20% chance to raise/lower luck by 1d3 points).
Equip: Ring of the Sand Djinn: +2 AC/saves, Invisibility for 1min/spellburn point, or unmake for great, unknown effect. Padded Armor, Longsword, Longbow, quiver w/20 steel-tipped arrows, 10 silver-tipped arrows, backpack, spellbook, quill and ink, sturdy parchment (10 sheets), 5 days rations, high leather boots, belt w/ belt pouch, gray robe, dark gray hooded cloak. Also carries 3 small mechanical toys: wind-up mouse, wooden puzzle cube, small jewelry box that plays a little tune.