With the right spring, I'd say you could get up to 800 - 850 FPS depending on the spring used and the quality of the tune. With an 18.3 gr JSB Exact, that would be around 20 FPE. How much power do you need? The "sweet spot" for most magnum springers is around 750 FPS for .22 and perhaps 850 FPS for .177, IMO.
I have been up and down the springer power spectrum, from 6-8 FPE up to around 22 FPE in a Diana 48 .20 cal with a custom tune. The Diana was a beast. Heavy to carry and hard to shoot accurately, but a real thumper. Want to head off to the woods and squirrel hunt with springers? If you go with me, I will likely be carrying a Mike Melick tuned B-26 .22 or an R9 .20 cal with a Maccari spring and seal, and shooting JSB/Polymag Predator pellets. As for taking small game, either has more than enough power and both are a pleasure to shoot. Both are shooting around 17FPE, I think.
You should ask Mike Melick or Gene Curtis for particulars about tuning the R9. Both can give you more first-hand information about what is possible with a good tune.
But you should decide for yourself. One super-magnum springer was enough for me.