paint to bore match will minimise curve balls, also keep the inside of the barrel clean with a fluffy or similar, that will cut down breaks due to residue in the barrel.
I know a problem I had was that I use to mach the bore size to tight. It would take a real peashooter puff to blow a ball down the insert; a noticeable result was the paint would sidewind and spiralling all over the place. I find the best way to match is to be able to blow a ball through the insert with minimal (but some)resistance.
I've got a full stainless freak set and it's brilliant. If you can't afford the full set then I would do as beanbag says and just get a couple of different size inserts