[Woodz428]

You might want to check with Dan Mroz, he's an SPF owner that also has a chassis dyno. When I talked to him last he indicated that unless you went pretty big that you wouldn't really notice a lot of difference in HP...maybe noise, but HP didn't seem to be impacted until you started getting pretty big. I'd talk with him about what you want to do and whether he feels the modified sidepipes will give the results that their costs justify.
He has run several cars with the stock pipes and modified/stainless ones for comparison, so probably has as much first hand data as anyone.