Not trying to side track things here, I have a Sniper system, so NOT familiar with fitech. But I had the sniper “dead headed” from the external pressure regulator like you due to being told the built in regulator would not handle the extra fuel pump volume. Holley and others said it would work. I had an issue I chased with a lean stumble on light throttle changes. It was commanding a ton of fuel in the problem area. I got to the point of putting a carb on. As a last ditch effort I pulled the guts out of the built in regulator ran full pump volume to the tbi with a return to the external regulator set at 58psi which is what Holley calls for. It cleared up every issue I had, went from commanding 80-90 pph to 25- 35. Also you didn’t mention camshaft, 13 degrees initial seems awful low with any type of performance cam. As I stated this was my experience with the sniper.
