The maximum is 100%
A 20th level Shadow bloodline sorcerer with the Solid Shadows metamagic feat can obtain 100% effects with their [Shadow] conjuration and evocation spells.
Shadow Master (Su): At 20th level, you can see perfectly in natural or magical darkness. When you use shadow conjuration or shadow evocation spells, your creations are 20% more real, and any creatures you create gain the benefits of the Augment Summoning feat.
Solid Shadows: When casting greater shadow conjuration, greater shadow evocation, shadow conjuration, or shadow evocation (and, at the GM’s discretion, any similar spell with the shadow descriptor that mimics a spell of another school), that spell is 20% more real than normal.
The fetchling's arcanist favoured class bonus (Blood of Shadows) can increase this effect by another 2% per level, but it is still limited to 100%. However, that is a better alternative to waiting until the 20th level as a sorcerer.
When the arcanist casts an illusion (shadow) spell that deals a percentage of its damage or effect against nonbelievers, increase this amount by 2% (to a maximum of 100%).
This is for 3.5, so i's more like trivia, but in the Realm of the Fellnight Queen module, there was a staff that increased it by 10% per charge, possibly going over the limit.
In addition, the staff can further influence shadow conjuration and shadow evocation spells, whether cast from the staff or the wielder’s daily allotment of spells, without increasing the spell’s level. By expending extra charges, the wielder can increase the shadowy substance used to form such spells by +10% per charge, up to a maximum of +50%. Using the staff for this purpose does not increase the casting time of these spells.