To partially answer my own question in case anyone else was wondering now that I have had a chance to do some testing:
Power Build Up (I went with the +damage option from power mastery rather than Soul Mastery) does increase the heal of Soothing Aura for PBU's duration. The unslotted heal increases from 43 to 73 per tick.
Unfortunately, the defense boost when Hail of Bullets activates does not show on the attributes monitor so I can't verify if it is being increased or not.
Couple of questions regarding the effect of power boost from Souls Mastery on some DP/Pain powers:
1. Will PB increase the heal of Soothing Aura for the 15 second PB durations since Corr. SA is a ticking heal rather than +regen?
2. Will the Defense you gain when Hail of Bullets is activated be increased if you PB before you pop HoB?
Looking at City of Data I notices Aguishing Cry, HoB, and PB all has 120 recharge so some extra healing and +DEF would be nice with HoB's long point blank animation.