It depends. What do you mean by "clothes"? I have a -5C (+23F) rated bag and that rating is very accurate for me. I can use it down into the low teens by wearing my Capilene base layer (old Patagonia midweight), socks, liner gloves and a fleece hat, and my overbag (MEC Emperor Penguin) and bivy sack (BD Winter Bivy, which is just a very light bivy).

Without the overbag, I will toss my parka on top and that seems to work about as well. HOWEVER, that parka, which I am wearing in my avatar photo, is a TNF Baltoro down parka that is almost as heavy as my bag. It is designed for subzero weather. Plus, I have insulated pants as well, which I could wear in my bag if it got really cold. With everything on, I could probably sleep quite warmly in zero or even slightly lower temps.

So, the bottom line is unless you take clothes for very cold weather, being comfortable down to zero in a +15F bag may not be a realistic expectation.