Stamina for keepers is more like 'concentration' - That is how I undertand the skill for keepers.
I.E Stamina for outfield players means that players can play to a higher level towards the end of the game.
Likewise I believe a keeper with good stamina, will be better towards the later stages of the game than one with lower stamina.
Goalkeeping is roughly 50% of a GKs skill, then ST/BC/PI/SP are alll about equal, then aerial passing etc.
Don't sacrifice keeping skill for stamina though.. how many balls in keeping does he have?