The HKS Dual Dragger came about because of some ground clearance issues with the normal Super Dragger muffler on some cars, also at the time, the dual canisters were the "in" thing... most of the big-name manufacturers had their own "Dual type" mufflers. I wouldn't think performance would be any worse than a usual Super Dragger exhaust system, although it'd be a bit heavier.
If performance is the important thing, the only thing I'd do is sell the Dual Dragger and get a Titanium exhaust (any brand)... the weight saving alone is worth it IMO.