I'm not convinced shard of freedom needs a nerf, but another possibility would be to make the +25% multiplicative instead of additive. This would mean that no matter how many shards you had in play, you wouldn't be able to achieve that 100% evasion, though you could come very close. The math would look like this: (whereas currently now it is 1 shard = 25, 2 shards = 50, 3 shards = 75, 4+ shards = 100) This would also mean that copies of shard of freedom drawn beyond 4 are not completely dead cards.
1 shard: 25
2 shards: 43.75
3 shards: 57.8125
4 shards: 68.359375
5 shards: 76.26953125
6 shards: 82.20214844
7 shards: 86.65161133
8 shards: 89.9887085
9 shards: 92.49153137
10 shards: 94.36864853
11 shards: 95.7764864
12 shards: 96.8323648
(you could get more than 12 in arena via steals, but it is both unlikely and would be fairly insignificant)
(formula is 1 - [0.75^x] percent evasion, where x is the number of shards of freedom)