I don't understand what you mean by massive loss in spellpower. As far as I know, most of the tier pieces are pretty on mark, and aren't that far behind a normal piece. As far as I'm concerned it's a very good bonus and I've never heard anything to the contrary. Are you comparing 277 heroic to 251 gear or something? Even 264 heroic/normal isn't that big of a jump and if you're getting that I presume you're able to get marks.
This is for afflic. Quote:
4pT10 gives a 15% chance of a 10% damage increase for 10s everytime UA ticks. The uptime of this buff is therefore ~41.7%, resulting in a 4.17% damage increase and making 4pT10 a must-have as expected
Destro Quote:
The 4p bonus is incredibly good and should be gotten as fast as you can. The 2p, not so good. But it's still better than the 4p of T9 so switching from 4pT9 to 2pT9 and 2pT10 is a very valid choice