I am in a small guild so I don't often get to do 25-mans, but I did do 10-man Naxx on Monday. Here is results (see
http://wowwebstats.com/fq46fau2oibtg):
Anub'Rekhan
Lifeward healed for 7630
physical damage: 470,689 x 0.254% = 1195
Grand Widow Faerlina
Lifeward healed for 3873
physical damage: 269,361 x 0.254% = 684
Maexxna
Lifeward healed for 7998
physical damage: 288,889 x 0.254% = 734
Noth
Lifeward healed for 6688
physical damage: 151,089 x 0.254% = 384
Heigan
Lifeward healed for 2718
physical damage: 8,154 x 0.254% = 21
Loatheb
Lifeward healed for 1713
physical damage: 124,084 x 0.254% = 315
Patchwerk
Lifeward healed for 8229
physical damage: 599,387 x 0.254% = 1522
Grobbulus
Lifeward healed for 9876
physical damage: 490,059 x 0.254% = 1245
Gluth
Lifeward healed for 7544
physical damage: 349,349 x 0.254% = 887
Thaddius
Lifeward healed for 7704
physical damage: 288,222 x 0.254% = 732
Instructor Rezuvious (no damage or healing due to mind control)
Gothik
Lifeward healed for 7121
physical damage: 100,523 x 0.254% = 255
The Four Horsemen
Lifeward healed for 7535
physical damage: 130,452 x 0.254% = 331
Sapphiron
Lifeward healed for 13,746
physical damage: 785,947 x 0.254% = 1996
Kel'Thuzad
Lifeward healed for 6647
physical damage: 227,414 x 0.254% = 578
all fights, all trash combined
Lifeward healed for 209,826
physical damage: 7,332,874 x 0.254% = 18,625
As you can see from the above its not even close. Lifeward heals roughly 10 times as much damage as the agility enchant would have mitigated.
Note that in Naxx 25 you take more damage but not huge amounts more. For example, in the run at
http://wowwebstats.com/nmhwqwwjxpgp5?a=x12a4256 you can see the main warrior tank took 7,322,874 physical damage. So the results will be similar to 10-man naxx.
Also, note that the heals shown above are only real amounts healed, overheals are not included.