MAP sensor should be as accurate if not more than a boost gauge, as you want to read the manifold pressure, not the output of the turbo. You have all sorts of restrictions and pressure drops between the turbo and the manifold, most of which is the intercooler and piping. The engine only 'sees' manifold presure, turbo output pressure is fairly irrelevant unless you are looking at the intercooler efficiency. Without taking my engine cover off to look, there should be a small (around 6mm or 4mm) vacuum hose connection coming off the manifold. As long as its going to a static source, you should be able to tee into it. By static i mean a dead ended sensor or something. If it actually flows air thru it then the pressure will not be accurate. Also make sure the manifold end is a direct connection to the manifold, and there is no valves or anything like that in the way. Hopefully there is something there for you - otherwise the Scangauge may be the best bet. Just remember as someone else said - the output is in absolute pressure, which takes atmospheric fluctuations out of the picture. Normally its around 14.7 psi, but can vary depending on the weather and altitude.
