Summoners and shields

[Lawstead]

I've made done a bit of tinkering these pat few days, and picked up on the fact that Summoners can wear mithral breastplates without penalty, ignoring proficiency penalties by either spending a trait or adding comfort property to the armor.

But what if the Summoner were to wield a mithral light shield, which has an armor check penalty of 0, and a arcane spell failure chance of 0%.

Would the Summoner then incur 15% spell failure for wearing the mithral breastplate?

[Avianfoo]

Wear mithral breastplate without medium armor proficiency? Lets have a run down:

Summoner wrote:

Summoners are also proficient with light armor. A summoner can cast summoner spells while wearing light armor without incurring the normal arcane spell failure chance. Like any other arcane spellcaster, a summoner wearing medium or heavy armor, or using a shield, incurs a chance of arcane spell failure if the spell in question has a somatic component.

Looks good so far.

What does mithral do?

Special Materials wrote:

Most mithral armors are one category lighter than normal for purposes of movement and other limitations. Heavy armors are treated as medium, and medium armors are treated as light, but light armors are still treated as light. This decrease does not apply to proficiency in wearing the armor.

...
Spell failure chances for armors and shields made from mithral are decreased by 10%, maximum Dexterity bonuses are increased by 2, and armor check penalties are decreased by 3 (to a minimum of 0).

So a mithral breastplate would be:

Cost: 4200 gp
AC Bonus: +6
DEX Bonus: +5
Armor Check Penalty: -1
Spell Failure: 15%
Speed: 30 ft.
Weight: 15 lbs

What does non-proficiency do?

Armor Proficiency wrote:

Normal: A character who is wearing armor with which he is not proficient applies its armor check penalty to attack rolls and to all skill checks that involve moving.

Summoner in mithril breastplate: Can cast spells without arcane spell failure chance, however they will take the -1 to attack and and movement related skills. This can be circumvented by using comfort armor or Armor expert trait.

Summoner using a mithril shield (or buckler): Would have 0% spell failure and suffer a non-proficiency penalty of 0.