Stamina penalty for armor sets and individual items of armor.
Recovery rates are tested empirically, please see method below.
* = tested
Stamina recovery rate: ~39 per second
Stamina recovery rate with Eternal Warrior's Ring: ~47 per second
Stamina Penalty: 1 Stamina Penalty = the amount of stamina you recharge per second goes down by .6
Additional information and data posted on - Atlus forums.