Not inaccurate. Just potentially slower response, and probably on the scale of hundredths of a second difference. It wouldn't matter at all for the boost gauge. For the BOV, as long as it responds quickly enough to avoid compressor surge, then the T fitting for that is fine too. A dedicated vacuum source for the BOV might possibly keep the turbo spooled a bit better during gear changes, but whether or not the difference is worth the effort of creating a dedicated vacuum source would require testing and data logging to determine.