Yeast will consume simple sugars first. As Maltlicker noted, ensure you have are using the proper pitching rate for the gravity. The yeast will consume all of the simple sugars and leave nothing behind but C02 and Ethanol.
Sugar does not make a less sweet beer. It makes a beer with less body and more alcohol. These throw off the flavor balance so you should understand the mash profile to ensure the beer stays balanced. Care should be taken to avoid this, watery beer. So, if you mash at 148, you might want to mash at 150.