In the end you can only do what you can do. Like getting into turning, the main tool (the cyclone or the lathe) is not always the bulk of the expense. It is easy to spend on ducting and fittings what you spent on the DC itself. This can be aggravated by your location and access to ducting. I have a 7" inlet and not doubt, a 7" main would perform better than my 6" main. The performance is good enough that I have not been driven to make that change yet. Going to 6" at the tool (from the often supplied 4") can put off this expense as well. It is not the 7" main so much as it is the fittings that kill your wallet.
"A hen is only an egg's way of making another egg".
– Samuel Butler