If you have a fluid trainer, the spacing between gears on a 12-30 is going to really mess with your trainer efforts. If you have a magnetic trainer, then you'll be okay. You don't get speed by varying the cassette. What you get is closer gear spacing. FWIW, I consider the 12-25 to be your basic training cassette. It's a good compromise. Pair it with a standard crankset if you are relatively strong, a compact if you are not.
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Cat 2 Track, Cat 3 Road.
"If you’re new enough [to racing] that you would ask such question, then i would hazard a guess that if you just made up a workout that sounded hard to do, and did it, you’d probably get faster." --
the tiniest sprinter