A 12-30 isn't going to slow you down appreciably IMO. On the other end of the spectrum, the 11-28 only offers one more "big" gear than your 12-30. Do you find yourself running out of big gears as it is with your current set up? Admittedly, I have run out of gears with my set up, but that's partly a function of my compact crank, not just my cassette. Plus, it generally only happens on downhills. So, I just coast; it's not a big deal since I'm not racing anybody.