As stated most flylines cores have a breaking strenght of 30#. The excpetions are the RIO leviathan lines. They have a core strenght of 70#. The reason for the high breaking strenght for backing and fly lines is not just about the breaking strenght but the strenght necessary to sustain a extended fight with a true big game fish (tuna and marlin). And most guys I know, unless they are fishing for an IGFA record, use much heavier leaders thatn 10kg. When fishing for BFT I usually use 40 or 50# flourocarbon leaders. So you darn well better be using at least 50# backing and preferrably 65# which is what I use.
I realize this post in ancient but came across cb3fish (or is it cb3fishmaster?) jumping this poor guy [WR] for a perfectly legit, and probably unknowingly good question. If cb3PO has ever fished for GTs or fished on any remote flats with ridiculous coral heads, or the fl keys ocean side with coral heads, crab traps, etc, he would realize the need for 80-100 lb backing. You can back off the drag when you see your fly line in trouble, but it's not so easy to see when you have 200 yds of backing out. And if you see the trouble and back off the drag on a GT, your headed for a yard sale. [WR] - stick with 80 lb (power pro is fine) and send cb3PO a pic of a trophhy that wasn't landed in 500 ft of water!! Airflo has a GT line that's 50 lb.
January 2013 I took a group of 8 to Christmas Island. The group did a fair amount of GT fishing. We lost 9 fly lines.
January 2014 most of my anglers returned with 80lb. backing and Rio Leviathan lines (70lb. core). We lost 3 fly lines.
I’d be the first to admit that this is a highly specialized situation, but most Trevally live near reefs and even though beefy backing and fly lines may exceed the lifting power of any fly rod, they may also survive several trips over under and around coral heads. This still usually results in lost fish, but often lines are retrieved and damaged backing can be removed and lines reconnected.
I would also submit that since capacity is so excessive on most big reels, the added diameter of 80lb. backing can be nice simply to fill a reel more quickly. As an example, I just filled a Hatch 12 plus with 450 yds. of 80lb. GSP. and a 500 grain line. This was the ‘large’ arbor version of this reel so the ‘mid’ arbor version would have fit even more. Had I filled the large arbor version with 50lb. GSP I would have been looking at somewhere close to 1,000 yds., which would have been excessive in an entirely different way.
My larger fly-reels are filled with 80lb Tuffline XP. Mostly because I'm cheap, and have a 2200yd spool laying around that I use for conventional reels. 80lb breaking strength may seem excessive, but as Anil pointed out, contact with rocks or coral can change "excessive" to "marginal" in an instant. Beside spool filling concerns, the diameter is easier to handle than something with lesser breaking strength. That can be a safety factor when fighting fish like tuna that can take line at nearly 40mph. At those speeds, a thinner braid could cut your finger like a band saw.
My camera had a seizure, humidity I guess, so I have squat of my own, except for Anil landing a yellowfin that took 81 minutes. The only pictures I've seen are on the PSFlyco facebook page. I don't do myface,but I saw them on my daughter's screen.