I don't mind throwing mid sized flies on an 8 or 9wt all day long. Flies in the 5-7" range are easy enough to cast all day long with proper double hauls.
I think that when you start getting into the musky sized flies (12-16") though, you will deal with more fatigue. Over time, however, you'll develop the muscles that you use for casting those bigger flies. The other thing to realize is that with more practice with bigger flies, especially musky flies, you'll pick up on some little tricks that will help make casting easier. For instance, I have a much higher release point on my 10/11wt cast than what I do when I cast an 8 wt. The cast generally won't be as accurate, but if I'm blind casting anyways, it's not usually a big deal. When I need a more pinpoint cast to a specific undercut, rock, or log jam, my release point lowers.
I also avoid false casts with heavy lines/flies. A 450gr sink link and a 14" double beauford are not easy flies to cast. For me personally, most of my casts need not be any longer than 30-40'. That's a distance that with one good haul, I can place without much stress on my casting arm. Avoiding false casts is probably the #1 tip I would give to anyone that wants to throw big flies.
And as always, making sure your rods/lines are sized correctly for the situation is key. With the new technology in the rods today, stepping up one or two rod wt's isn't a big deal in terms of the weight of the rod. Maybe a few ounces? And just be realistic about the size of fly and line that you can handle at your skill level. Casting 400 or 450 gr sink lines and big flies takes lots of practice, and is frustrating. If you don't have a proper technique with a 5 wt, it will just show in an exaggerated way with a heavier rod.
I can't comment on tarpon or other salt applications though since I've never had a chance to do it. But most of those flies are smaller than the flies that I use, so I would assume my advice would be transferable, although more precision is definitely required.
