Siege Masters Discipline "Increase the power of your At-Will powers by 11%, this is double in stronghold". How i tested.
I unequipped all gear and companion bonuses apart from daggers that had no bonus effects.
I tested on target dummies
I play Rogue and tested using Cloud Of Steel and Sly Flourish.
I equipped a companion bonus that wouldn't effect damage output(level 750).
Results from Cloud Of Steel and Sly Flourish:
Normal hit - 1313
Deflected Normal hit - 889
Deflected Crit - 1366
Crit - 2019
I then unequipped the companion bonus that had no effect on damage output and equipped Siege Masters Discipline(also level 750).
The damage output was exactly the same. It is supposed to increase At-Will powers by 11%.