Soo I'm a little stuck here...the gfs-10 fuel flow meter has 3/8" hose barb connections, and the boat has 1/2" fuel line. Will it cause any serious flow issues if I reduce down to 3/8" for the sensor then go back up to 1/2" after?

Did you ever connect the garmin flow sensor to your hp500? I have a 575sci and would like to do the same but have the same concerns