250-500cc 2 Stroke back pressure

How will the amount of back pressure from the exhaust affect the performance of a 2 stroke?

Less back pressure tends to improve low end response, more improves top end. And that's a pretty generic explanation.

"Back pressure" is sort of an outdated/vague term. 2-strokes (and 4 strokes, although in different ways) depend on waves bouncing and resonating inside the chamber, so the static pressure in an exhaust is not the complete picture.

There are lots of "urban legends" about "backpressure" with regard to 4-stroke performance (more backpressure means more low-end torque, etc), but the reality is that it's not really backpressure that is having an effect, it's wave scavenging. Static backpressure is almost always a bad thing...

I agree, but my reference is totally based on 2 stroke and "stinger" size. But, as you stated, it's really it's effect in scavenging. The term "backpressure" is very generic, loosely used term from way back in time. When that phrase was coined, people didn't really understand what was going on and simplified it to the term "backpressure".

I thought I understood what he meant by his general question, hence the "generic explanation" disclaimer on the end.