+1 for a warmer pad if you haven't already maxed that out. Thermarest xtherm or the new Nemo extreme conditions tensor will make a MASSIVE difference in how warm you sleep.
+1 for a bottle of hot water in the foot of the bag
But to answer the main question: I've found draping down layers *over* the bag to be much more effective than wearing down layers *in* the bag, but it all depends on the specifics of your setup. If either layer is compressing the other, you're losing out on potential warmth for the same weight.
I should say that I sleep warm, at least relative to my wife (who uses a 9°F bag year-round), but in my experience you get a lot more versatilty and bang-for-your-buck/lb by finding a way to incorporate the warm layers you use during the day into your sleep system vs buying a bigger, bulkier, heavier bag.
I own a 20°F bag and a 35°F quilt, and it's been a long time since I've reached for the 20° even in sub-freezing temps.
As always, YMMV.