Moving from DSLR to CCD?

[MG01]

As I get back into AP I'm considering moving from my 5D mkii to a dedicated mono astro camera.  I had a read through the excellent pinned post but one thing I'm finding hard to get my head around is how to compare one with the other.

I understand that it is about more than just megapixels and that the smaller FOV of an astro cam can effectiviely give me the same resolution on target as a higher MP count DSLR.  But what I'm trying to get my head around is finding the point at which I POTENTIALLY can achieve better results than my DSLR. 

I understand that there are many variables, but hopefully someone has some advice to guide my decision making processes.

Thanks