I installed a muffler on mine to protect my kids hearing but I absolutely loved the distinct sound of my Whistler. I went 190 miles each way to a few GA Rallies and it never bothered me. I put the muffler on while my truck had a D Turbo on it and I could hear a little bit of spooling. When I put the C on, it'd scream exhaust note wise. Performance wise, research shows the D flows better. I didn't notice a difference. If anything the C may have tricked me into thinking it had better low end power due to the audible experience. It is a 134 HP engine in a 13,000 pound truck so 5 HP likely wouldn't be that noticeable. I did pull a 22,000 pound MK-18 Ribbon Bridge Launcher trailer meant for an MK-48 one time and could maintain 55 mph on flat ground and 40 mph at the slowest in 4th gear on hills with the C. Yes, the towed trailer weighed more than the prime mover but the brakes on the trailer worked so well that I don't think my brake shoes ever touched the drums.