As in all things, there is no one answer for all
As Ed pointed out, the performance benefit of bigger mufflers is somewhat dependent on displacement. All SPF side pipes are the same, so increasing the muffler diameter has more benefit for 500ci’s than it does for 400ci’s. That said, it is not only the muffler size that hurts. The SPF side pies also have some built in restrictions, particularly in the collectors. While the big blocks see more advantage than the small blocks, even the small blocks will get an increase in power from improved side pipes.
As for the headers, this is more dependent on what heads you have on your small block. SPF supplies the same headers for all Windsor style engines, while all Windsor exhaust ports are not the same. The SPF headers fit the stock 351W head well, but if you have the FRPP “Z” high flow head or equivalent, there is a miss match between the head and header that will cost you some flow,
Ron Roberts, Firefly Performance Products,
www.fireflyperformance.com has been working on this for the last six years and has developed both stainless steel side pipes and headers to address the issues. I worked with Ron on his first set of side pipes for my SPF with 514 and I got significant increase in both horsepower and torque. Here on the Firefly web site you can see my dyno sheet
http://www.fireflyperformance.com/Pipes/index.htm
Of course performance is not everything…. These Firefly pipes are knock out gorgeous and sound fantastic too. Take a few minutes to peruse
http://www.fireflyperformance.com/
Craig