Generally, I rely on clothing layers only as a safety margin for the sleeping bag when conditions get colder than forecast.

I pick the bag to meet the lowest temperatures I expect to encounter. If my trip projects lows in the mid-40's, I take my 40-degree bag; if lows are forecast between 20 and 40, I take my 20-degree bag; lower than 20, I generally stay home.

This strategy worked very well for me one fine January evening, when the lows (predicted to be 20) ended up being zero. Fortunately, my bags are rated about 5 degrees conservative side, which got me to 15. Midweight longjohns, down pants, down jacket, down vest, down mittens, and down booties) kept me comfortably warm to zero. Of course, two other critical factors were an adequate mattress (Thermarest with an R-value around 4) and the fact that my bag was cut wide enough that I could wear all that clothing inside it without compressing the down in the bag or the clothing.

While many others may not agree with me, I don't see "Take a warmer bag and rely on my clothes to make up the difference" as a valid lightweight strategy. The risk seems disproportionately high relative to the 3-ounce reward of the lighter load.

I can't fault you if you come to a different conclusion - provided you're aware of the issues. I'd maybe suggest that, if you do go the route of relying on your spare clothing, it might be best to do a test run camped at (or a short walk away from) your car, in case it doesn't work as well as you hope.