Why don't you save us the trouble of buying them and finding out that they are no good by telling us EXACTLY what the problem is with them and how you know?!?!
im not saying they are no good at all,but they sure has to create restriction from turbulence considering the design. i have looked at hundreds of X-Pipes of different brands and types and made quite alot of them aswell. the problem with the pressed sheetmetal type X-pipes is that they all have tubefittings stuck into the X-part that is almost like a chamber these fittings are not welded edge to edge with the sheet x section the tube fittings create a step. and therefore must be a restriction. might not be a problem at the inlett side but sure has to be one on the inlett side.
think of the X section as a piece of 3" tubing with the fittings being 2½"tubingstuck into the bigger tubing and then a fat weld around them. then look into the bigger tube and you will understand what the exhaust pulse will look at trying to exit these xpipes.
i suck at explaining stuff like this. much easier being able to point at the problem with a product in hand.
the only reason i can see for that designs popularity is that its cheap to make and very universal for a manufacturer.