Perfect, thanks. That's the missing bits of information. Definitely see what you mean about tall pistons. Not necessarily the worst thing for a turbo engine that likes good thick ringlands, but I can see the potential for improvement with longer rods. Considering those dimensions your 476g pistons are remarkably light! The first comparable piston I can think of is a Mitsubishi 6G72 piston which are about 1mm too short to use with Ghibli rods and the 67mm crank (31.6mm compression height, 92mm bore). JE forged ones of those are 432g with pin (347g bare, not sure if rings are included in the weight but they won't be more than 5g).
I wouldn't necessarily advise using those particular pistons though as they're dished so won't help the compression ratio (unless the Maserati pistons are also dished), but they're very similar bore and for a turbo engine so should be indicative of what weight reduction you could expect with shortening the compression height. Not nothing, but not night and day either (comparing forged to forged).
And yeah, 21.5-23.5mm consistently across 12 ports is good going! Enabling consistent work across ports is one of the major benefits of a flowbench I think. Make one good port, then you can incrementally creep up to the same flow on all of the rest. It's no substitute for careful workmanship, but it's useful to have some form of benchmark.
Edit: A quick google suggests the Maserati pistons are dished, though I can't easily tell what volume.