How much of a difference you'll notice will depend on how often you currently use your 30 cog. If you don't use it much, then you likely won't notice it at all. If you use it a lot or some, then you might, but the gear difference between a 28 and a 30 is pretty small. However, if you already feel like you are running out of gears on your 12-30 setup, then the 11-28 is arguably not your best choice.

If you want to know the exact difference between the 30 and the 28 cogs, use Sheldon Brown's gear calculator.