I never add as much extract to my starters as I'm supposed to, but they still work. Yes, you'll need to take that much extract out of your main boil to compensate.
Or, alternately (and optimally), you can cold-crash your starter the night before you brew by putting it in the fridge. Once the starter is fermented out, you've created the requisite yeast cells that you set out to create. Now all you have is uncarbonated, unhopped beer...so what you do is cold-crash it in the fridge for a day or two. The yeast will mostly drop out of suspension and leave the beer above. Once the separation is clear, you can decant and discard a good portion of the beer, then just pitch the yeast slurry. This way, you minimize the amount of starter DME that's going into your beer.