So many things to think about with mufflers. One is how do they sound, second is how much HP do the cost. Below is a dyno run of a late 911SC track car with headers and straight thru glass packs on the end of headers. We dynoed the car could not believe how little power it made. We convinced the owner to pull the glass packs. The were of a design similar to this, straight thru:
apparently those bump outs into the exhaust stream really hurt below is the with and without glass packs.
This muffler cost 13HP and 11lb ft of torque on a motor that only made 159HP and 150Ft/lbs on straight pipe, so about 10% loss.
john