could be a fault like the imrc playing up which is very common fault with the st170's
could have a sensor down etc worth having a look at.
they also suffer from cat and lambda sensor faults also check maf unit
might also be worth running it on super unleaded
the kn systems are very good and should of seen some gains above std power
as for the peco unit is full system in stainless steel or mild or just a back box ?
the mongoose/blueflame +miltek systems seem to give good gains of power
if not happy with the car get it looked and set up properly (chipped) etc
each dyno is different to others tho so will vary in results