Not game-breaking, but game-slowing. Add the Margin of Success (MOS) to your missiles-hit roll.
For example, you need to roll 9 to hit with your LRMs, and roll 10. Add 1 (10-9) to your missiles-hit roll.
If you needed 5 to hit, and rolled 11, then add 6 (11-5). Simple, but an extra maths step. I'd do the same for Ultras, but possibly not for LB-Xs (which are cool enough anyway).
Yes, it makes missiles more effective, but LRMs still hit in 5-point lumps. And how often are you firing on 5s to hit? ;)