Dan,
You might try doing a web search to learn more about tuned pipes and how they work. You are basically correct in that you vary length to optimize a particular pipe to a specific application, but other factors also influence it. Some pipes work better than others on specific applications because rpm, engine displacement, engine port timing, pipe volume, internal geometry, fuel, weather, etc. all affect tuning.

The idea is to set up a pressure wave inside the pipe so that it scavenges the exhaust giving a supercharged effect. Most DF pipes on the market are pre-tuned to work well on a OS91/Dynamax. They may not be optimized but usually work well on similar sized engines operating at high rpm.

Most tuned pipes you see are full wave pipes (one scavenging wave per exhaust event). Mini-pipes are half wave pipes (two scavenging waves per exhaust event - so one of these waves is essentially wasted since the exhaust port is closed when it hits it). If everything else is equal a full wave pipe will perform slightly better than a half wave pipe). Weston seems to have done an exceptional job of mastering the science of half wave pipes and in many cases they perform as well or better than full wave pipes.

If you do a search in the Jets Forum you will find many who are amazed and very pleased with the performance of the Weston DF Mini-pipe.

Joe