Guys, the Wren pipe doesn't converge, it diverges. From the bellmouth back a certain distance the pipe diameter increases. Then from that point back it's a constant diameter. Most other pipes converge slightly throughout their length. I have also seen some that are a constant diameter.

To answer your question whenever you flow a liquid or gas through a pipe the resistance increases with length. How much it increases is dependant on a number of factors. The most critical factor is pipe diameter. In this case as long as your pipe diameter is sized large enough for that engine and you are not making the exit diameter any smaller I doubt that you would detect any noticeable difference in thrust.

For example, as long as your pipe is of sufficient diameter that it isn't starting to choke off the flow and you replaced a 24" long pipe with one 30" long, keeping the same inlet and outlet diameters, then any change in resistance or thrust loss would be neglegible. If you have any doubt you could go with a slightly larger diameter for the longer pipe. I'm sure that if you contacted any of the custom pipe makers in the hobby that they could advise you accordingly.

Joe