It probably should be noted that when you're dealing with the heavier weights (I can really speak to that since that's what I fish the most) that materials matter. I think you might have issues dealing with a size (let's just say 2 @ 4" long) long weighted rabbit strip streamer with your 6 or even 7wt, but that same fly tied with say, craft fur or bucktail or some other material that doesn't soak up water, you could probably cast it easily cast it with perhaps even you 5wt.
bumping up even further, there are some of my tiger musky flies that are in the 8-9" range that absolutely suck casting on even a 10wt. Put them on a 12? it's too easy. Whereas, I have some flies that are in the 12" neighborhood that cast super easily on my Bandit (10wt) or NFC 10wt because of the appropriate use of synthetics.
Bottom line, it depends. fly construction, angler skill, line, specific rod (e.g. not all 6wts are created equal).....oh, have fun with this journey of emptying your wallet of your hard earned cash.