Bill:
I was just referring to.22cal, but the 350 starts off at 835-860+fps with 14.x pellets. I have also read posts from 2 different people getting 720-730fps with the Arctic spring but I think those people might have a breach seal problem and they need to shim it or replace it or something else is going on. I could be wrong however, but the mid 700's seems to be the norm. I know people that have a Macarri GRT kit in their 34's getting 710fps with 14.3gr CP's or more with some spacing. If I wanted 700's I 'd go with a different gun.
The 350 was made to handle >900fps (.22). What the 350 needs is a good metal (delrin or naval brass even better) spring guide and top hat on top of a good lube, debur & polish. It's the plastic rear guide that causes most of the issues found in most Springers, ie: the torquing and vibrations. Also add a custom/heavier top hat and it will handle heavier pellets no problem. Speak to people that have done a good tune w/custom guide & tophat but stayed with the factory spring (or new factory spring). You 'll be surprised.. or contact a tuner like John-in-PA.
Again, there 's nothing wrong with a smooth less powerful 350 using the Arctic spring. It will make the gun more accurate and pleasurable to shoot, but sometimes a Maccari spring is not enough and doesn't solve all your issues with the gun. The 350 can be so much better with a good tune & proper lubes. Don't forget these are massed produced guns. The metal is mass machined and they can't spend the time inspecting and perfecting everything inside the gun without charging a lot more for it. Most Springers also inherintely have flows like the plastic rear guide which is meant to keeps costs down. With a long spring like the 350's that guide must be replaced with something sturdier to eliminate vibrations and torquing (the spring turning which also makes the gun turn in your hand when shooting it).
