Actually, I think that her shield amount should be nerfed pretty heavily (or the AD). I believe it's the strongest support shield in game. For comparison (the cooldowns are all the same):
Lulu - E:
Mana cost: 60-100
Range: 650
Shield Strength: 240 +60% AP
Duration: 6 seconds
Other bonuses: +9-105 (+15% AP) magic damage per hit (based on champion level, not skill level), but can be intercepted by any target
Karma - E:
Mana cost: 60-100
Range: 800
Shield Strength: 240 +50% AP
Duration: 4 seconds
Other bonuses: +40-60% Movement speed
Janna - E:
Mana cost: 70-110
Range: 800
Shield Strength: 240 +70% AP
Duration: 5 seconds
Other bonuses: +14-50 (+10% AP) Attack Damage
You see? Not only it gives huge amount of AD (that's a BF sword there - without counting the scaling, which can easily result in ~100 AD in late game) - it's also the strongest one.
I'd say to butcher it's scaling to a more sensible level, like:
Shield Strength: 240 +45% AP
Would make sense for a shield that gives so much AD to be the weakest one, not the strongest. I would also nerf the AD on earlier levels, to like:
10/20/30/40/50 +10% AP (from 14/23/32/41/50)