That's a pretty broad statement, and very, very subjective. Boron is a great material, but not without its own flaws. It's inherently more brittle than graphite, because of the short chain polymers. As a result of this, more material is used in blank construction, albeit it makes the blank more dense, not necessarily heavier or larger diameter.
Another aspect of boron filaments, which is what you're getting in a boron composition rod- I know of not a single pure boron rod out there... Is that the process similar to an aramid veil. The way the blank is constructed adds rigidity and strength, not so much sensitivity. You're still relying on the base carbon for that.
In regards to weight alone, boron is far heavier than carbon. In the specific case of boron matrix for rod building, it's heavier still because of the tungsten filament used for deposition.
Undoubtedly, the most effective use for boron matrix in a rod is to add lifting strength in the butt section. In all reality, that's all the intent ever was. Add strength to the blank low which allows you to move more material out of the blank elsewhere.