I couldn't really think of any reason it would require jet changing either, but everyone says it does, but I should add it's only been when talking about bikes. I don't see why the principles governing their engines would be any different though.

A question relating to the post above denny's: why would REDUCING backpressure REDUCE scavenging? I would think it would help scavenging.

Even so, if there is more EG remaining in the cylinder wouldn't it just act as inert gas since it has no more oxygen (or very little) so it wouldn't contribute to AF ratio.
The fresh charge still goes through the carb, just a little less or a little more. . .right.