If you go with a glasspack-style (non-baffled straight-flow) muffler, I can't imagine 4 of them will be that much quieter than 2.
I think you'll get some decent flak from the Porsche-philes for putting a glasspack on your GT3. Also, you WILL scorch the paint off the muffler body if you engine brake while racing...with no cats, the rich exhaust popping will heat up the pack. You're better off with a higher-quality, stainless/aluminum-bodied muffler.
I'm cheap, so the Dynomax, Kooks, or Vibrant appeal more to me for cost, size, and construction.