Apr 13th 2009 4:55PM The only thing that is 'retained' when SW:P is refreshed by Mind Flay are +damage buffs, such as Shadow Weaving. A +sp trinket proc will only affect the power of SW:P for as long as the duration of that proc. The +sp coefficient of SW:P is recalculated when it is refreshed by Mind Flay.
Popping an on-use +sp trinket when you first apply SW:P has no effect on it past when that +sp buff fades. The only thing you need to make sure is that you have 5 stacks of Weaving up when you cast your first SW:P.