I don't really see any build outside of the standard 57/14 that will give you any more crit or spellpower really. I sit at around 2300 SP raidbuffed (about 2400 with a flask running), and about 26% crit raid buffed. With weakened soul on the target, another 4% brings me to 30% easily. This is without a soul of the dead, which would give me another ~2%. I am factoring in a boomkin here (would also be true with an elemental shammy).
Crit has nothing to do with keeping Grace up; grace comes from any direct healing spell (renew and PWS don't refresh or place the buff) - so assuming you're constantly healing him (you have about ~8 seconds to refresh the buff) I don't see it falling off.
Higher crit would mean more DA procs, which is nice, and what I think you might have meant by grace was inspiration - which is also very nice to have 95%+ uptime on your tank.
I know I am personally looking for some more crit, as I'm close to optimal spellpower, and my crit could use some work (my haste is also fine at about ~17% raid buffed). I've got a
[Darkmoon Card: Greatness] coming as soon as the faire hits - which means I will get more crit; but I;m going to lose about 100 sp from dropping my
[The Egg of Mortal Essence] - until I can get my
[Illustration of the Dragon Soul] to replace my other trinket (
[Forethought Talisman])
I have also looked at
[Soul of the Dead] but I have yet to see one drop, and since I'm close to BiS with my greatness card, and waiting on the Illustration - I don't think I will end up picking it up.