Sure I'll explain these.
They can come with some damage increase, up to about 30%. This 30% does not count towards imbuing intensity, but the 30% does count as an additional property. So, if you imbued Damage on top of the damage that was already there, you can get to 80%, but then you only have 3 mods left.
It's interesting for sure, but probably the best you could come up with is maxing swing, 80 damage, then hit spell and max hci.. That's the closest you could reach to the intensity cap anyways, since you really only can imbue the 4 mods.
Typically, A of EXC style weapons go for 500kish max, so I can't imagine the price being any higher than that + imbuing costs.