Honestly, top and bottom bowing in is a regular problem on the MS-2930. I don't think you will solve it with just convergence strips (or a cap kit). You could try reseating the yoke, but it's really easy to make it worse.
As for left and right, before you do anything, adjust SPC.15 on the chassis. SPC stands for side pincushion. You can try adjusting SPC.31 as well, it will have an effect for 15khz as well.