Assume an Illumian can enter into Shadowcraft Mage, through its non-Gnome adaptation.
That Illumian uses the standard Earth Spell trick, and any spells they heighten are actually heightened at +1 level (Ie, a Silent Image heightened to 6th level is actually 7th level).
Illumians have Power Sigils as a supernatural ability. The sigils grant an Illumian 'certain bonuses'. The Krau sigil bonus is +1 caster level.
If an Illumian takes Improved Sigil (Krau), they can 'tap into [their] krau power sigil to augment the energy of [their] magical utterances.' Practically speaking, this translates to the ability to increase the effectiveness of Heighten Spell by one for a select number of spells (2 for Illumians above 1st level). That is, the Krau power sigil is now providing +1 caster level, and is allowing a spell (Silent Image, we'll say), to be Heightened by +1. Thus, our Illumian with Improved Sigil (Krau) and Earth Spell is heightening spells at +2 effectiveness (Ie, their Silent Image heightened to 6th level is actually 8th level).
Let's consider what happens if our Illumian now takes Enhanced Power Sigils. This feat causes '[t]he bonuses granted by each of your power sigils [to] improve by 1.'
What bonus is granted by the Krau sigil? Merely +1 caster level? Or is it now also improving Heighten Spell by 1? If it is the latter, then surely our Illumian, with Improved Sigil (Krau) and Earth Spell can now mimic 9th level spells with a 7th level slot.
On the other hand, if it is divorced from the Power Sigil, then it is not a Su ability. Thus, the Illumian will not lose it when Shapechanging.
Which is the correct interpretation?