I built something very similar to that once. On my version I was hoping to get sound dampening by making the cone of perforated metal sheet stuffed with packing- rock wool, stainless, etc. It did work- I thought it sounded good- but the packing got all blown out way too quickly and of course it got real loud too. I planned on keeping the housing but making a conical sheet metal shield for the perforated cone with the idea that the shield would take the direct blast of heat and pressure while the packed cone would still muffle with absolutely minimal back pressure. Never did that phase II.
Anyway, my impressions of it are all subjective as I didn't put it on the flowbench or test it with the sound meters. Additionally I didn't do any stopwatch/dyno testing to see if there was a power difference although subjectively I'd have to say the fart can with cone baffle didn't seem to make any difference vs. the stock muffler.