NRR: To the mechanics, does lack of an exhaust system hurt my engine?

I'm not a mechanic, so take this with a grain of salt, but before I even read your post I was going to tell you that you were going to get bad gas mileage becasue of a lack of backpressure (saw it on TV one time). Not sure about the engine damage.

I would htink any "chain" muffler place could have any exhaust part for you pretty much as needed. Maybe it's an old Tercel? Or are you putting a cherry bomb on there?

Yes, constipation will slow you down.

So,my brother the ASE certified mechanic tells me that this "might" damage your valves. Why? One of the purposes of that now non-existant pipe is to stop the back flow of air into the internal area of your engine. In summer heat, it's not as big a risk because the air temp is higher generally-ealy mornings might be a problem n some parts of the country. This matters b/c if really cold air gets sucked up internally, the reaction of cold air with the hot metal will cause your valves to warp-ouch! Keep in mind that this could still happen, and that would kill the engine dead because the cylinders would no longer hold pressure. No pressure, no compression, no spark, no go. So drive at your own risk. Avoid high rev driving where the engine RPM's go high, then you let off quickly, this will cause air to be sucked internally. Steady even throttling is what you want to avoid damage as much as possible. Very basic and simplistic, but you get the idea.