If you're uncertain, is it feasible for you to visit a shop that has an in-store tech and shooting range? The tech might have open springs you could test with so you'd know if an M100, M95, M90, etc would be the best option for you.
As otthers said, there isn't a practical linear way of determining the change in performance even when you know the spring as the air seal in the cylinder has an effect too. Sometimes there is trial and error.
Pardon my confusion but is that the weight where you're quoting a current speed of 350fps? I'm not sure that's even legal, let alone permitted on outdoor sites for a full-auto capable airsoft gun.
328fps is still sketchy with 0.25g pellets as that's above ~1.14J. Just to clear any doubt, what's the site's energy limit and/or what's the maximum speed permitted with a pellet of a specified weight?
FYI, it's not a universal rule that indoor/CQB sites don't allow ~1.14J - the site I play at does permit up to that energy limit with up to 0.28g pellets. I know the likes of Bunker 51 have a hard limit of 1J.