Gwilym, I think my main issue is that I simply haven't played that many cabs, so am less sure about how much more "punch" or "grind", or "tightness" or whatever other hard-to-describe descriptive terms, other cabs might have.
I am a lot more experienced with different amps though, so am fairly sure that further expenditure on an amp would be fairly pointless for me.
The other issue is that youtube-type soundtests (limited though they are) can give you a clue as to what an amp will sound like. But for cabs (especially at anywhere near gig volumes, where cab characteristics can change noticeably), they are pretty much useless.
As for more money always buying a "better" cab I know it is not necessarily so. I used to have a TC electronic RS cab, then bought a cheaper BC cab, but actually liked the BC better, so sold the RS and bought another BC.