I pretty much have the same set up as you. Walbro 485 and Tomei L FPR. Mine only goes down to 40psi with the vacuum on. It used to be 43.5 with and without the vaccum attached. I got rid of the hardpipes on the plenum and just used fuel hoses and it lowered it a bit. I'm thinking the lines are small for the pump itself? But if you read the article by Nick from ConceptZ he was able to go to base pressure. As per your leak, was the hole you drilled too big? Mined was a very tight fit and no problem at all. I'm following this post if for an answer as well.
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