the OEM midpipes only have flex setions, no resonators. Both OEM and Thermal have a sinlge resonator and a muffler on each outlet. This is what helps keep Thermal systems quiet compared to others.
OP, aftermarket exhaust will not do much in terms of gains. The only reason I changed mine is because the OEM flex sections rattled like crazy, the OEM welds were prone to cracking, and the fake tips drove me nuts.