The 28% rate will not apply to your entire
taxable income, only the portion above the bracket cutoff, which varies by filing status. Here are the 2015 brackets:
If you are married filing jointly the 28% marginal rate kicks in at about $151k. The effective rate is lower than the marginal rate because you're taking an average across all of the brackets your income falls into. The next bracket down is $75-151k with a marginal rate of 25%. You shouldn't see a 13% increase in your tax liability just from the raises.
The brackets creep up a little each year to adjust for inflation. This is what they should look like this year:
With an income like yours that is growing you should consider doing some tax planning with a qualified tax pro.