From my limited experience (only put programable efi on four cars and mapped it) i have found it to make absolutly no difference in map pipe lengths. As long as the pipe is all the same bore and is free from restrictions etc it makes no odds. I have only heard it is a problem if, say you are using the ecu on an aircraft or something where the ecu is mounted far from the manifold,where the pressure signal would have a delay while the pressure was building in the line. But for automotive use, where the map pipe is only a few feet then i see no problems. As for being too close, some map sensors (pug gti6 for example) are actualy mounted directly in the manifold.