how would these work out for stacking?Some additions:
Shield Focus [General]
Prerequisites: BAB 1+, proficiency with shields.
Benefit: Choose a type of shield, such as large or small. Your AC bonus from that type of shield is increased by 1. Also, you gain +1 to attack rolls with that type of shield.
Shield Specialization [General]
Prerequisites: BAB 3+, proficiency with shields.
Benefit: You gain double the base AC bonus from large and small shields. In other words, small shields now grant you +2 AC and large shields grant you +4.
Shield Mastery [General]
Prerequisites: Fighter level 8+, Shield Focus, Shield Specialization
Benefit: Choose a type of shield, that you have taken Shield Focus for. You gain the following benefits:
-You gain your shield’s AC bonus to your Reflex saves against effects that allow Reflex saves for half damage.
-You gain DR 2/- if you specialized in small shields, and DR 4/- if you specialized in large shields. You have this DR against any attack that your shield’s AC bonus applies.
-Your shield is considered 1 size large for the purposes of disarm attempts or how much damage it deals in a shield bash attack.
Note: Shield Specialization used to be called Shield Mastery. Also, I revised Improved Shield bash to give +4 to attack with shield bash attacks.