Since the PCM gets readings for vacuum/boost, I would imagine the DIC boost gauge would get its reading from there, but then I have no idea why it would be inaccurate.
Just install a standard boost gauge when you do the M90 kit... With a setup like that you'll want to know exactly whats going on. Plus it'll look cool
When you do the vacuum lines on the kit, you just have to put a T in the right spot, and run a line into your cabin to the gauge. Or go with an electronic sending unit so you don't have to run a pressurized line and worry about leaks.
-Bill