Some of these replies leave one wondering just how much offshore fly experience the posters have had. You should be plenty fine with 30# tippet for mahi. You can go up to 50#, but will have a difficult time keeping tight loops with heavier leaders, and it will hurt your distance, If you're not fishing for records, I'd just tie on a stretch of 30# or 40# to your fly line and have at it. If you get a marlin in your spread, try it with 6' of 50#, to keep from abrading on the bill.
If you have a good boat driver, he/she shoiulf be able to get you on top of the fish regardless oif your tackle, and then you have to work a bit. With anything over 100#, a 15 wt is a help but not essent6ial. Just point your rod at the fish and crank the reel. It doesn't feel too elegant, but works well, and the stretch in the fly line will keep you from breaking off. Keep as myuch bend in the rod as you feel is safe, but just keep crankiong the reel.
The most important thing with mahi is to have a variety of flies ready. If they 'catch onto' a particular fly pattern and just follow it to the boat, a drastically different fly will get themn to hit again. I've caught mahi up to 54# on a 10# and haven't felt overmatched. Just jerk on them a bit to get them to go airborne and wear themselves out.