I can't find any comprehensive charts for you measuring velocities from a .12 reference, I believe in great part because somebody with that kind of time and interest in these things wouldn't even bother (.20 is considered the baseline/ minimum standard by players and enthusiasts).
However, from only the first two entries in the top-left column, you can see how dramatically the nearly doubled weight of a .20 affects velocity vs. A .12g bb.
If your gun is shooting 315fps on a .12, you can infer that loading it with .20s will yield and fps value of somewhere near 200fps +/-. This doesn't mean it'll be completely ineffective (I've had fun in goofy/ challenge/ zombie games with all kinds of crap), but the bbs will be going reeeeally sloooowwwly. Adding hop (if your gun has a hopup) will slow them down even more, and your gun may not even be able to propell the bb fast enough to really be picked up by the rubber and create sufficient backspin to do anything.
Going to quality .20s (or heavier) isn't going to change your life in any way you'll appreciate, but it will show you how shitty Walmart/ Cabela/ Cantire .12s are.
Good player/ enthusiast grade bbs are perfectly smoothe, round, polished, and roll on a flat surface evenly because they nearly completely lack (or have carefully controlled) air bubbles inside them.
Experimentation with springers can be fun, and can teach you stuff, but the relative accuracy you achieve will be born more out of the jump in sheer quality than anything else.