I have always build my own exhaust ststems, and yes the silencers have a LOT of influence on noise AND sound.
The best ones i have tryed until now is simons, and they have both stainless and std steel silencers.
Both are wery good straight flow, and they have a LOT of different sizes to acommedate your needs, and they also last a long time before they collaps(stainless one)
I have build one for my vw passat 1,8Turbo in stainless steel, and with 2 silencers and no cat it still holds the limit at MOT, + it gave me 7bhp/27nm on the rollers.
Flow wise just dont do any 90` bends on ANY exhaust, better with one 45` little pipe then one 45`again, straighten it out as much as possible, then the flow will come by itself