I've done several flights that have consistently delivered 25 minutes with a pair of Zippy's. Actual times ran between 0:25:06 and 0:25:19. That was taking them down to 21.4 and 21.3v. The Graphene delivered 0:21:12. Using three brand new stock batteries I obtained 25 minutes just once. Every flight using them afterwards has delivered progressively less flight time, with max flight time using Yuneec batteries now down to about 17-18 minutes or so. FYI, that's over the course of only 30 flights or so, with some of those flights using the graphene and Zippy batteries.
Just so you know, I use 21.4v as my general landing voltage. You can go a little lower but the 920 can initiate an auto land below 21.3v. As a rule I don't usually strive to obtain the most possible flight time with every flight. The long flights are generally performed during battery performance tests.
Short version is IMHO a pair of Zippy Compact 5000mA or similar batteries provides better flight time, performance, safety, and value over the factory batteries by an overwhelming amount. So much so I just bought another pair. They are also superior to the 8000mA graphene battery in flight time and safety as using a pair of Zippy 5000 batteries provides margin for a single battery failure that would doom the aircraft if using only a single 8000mA battery. Cost difference between a single graphene and a pair of Zippy's is only a couple bucks. $118.00 for one versus $120.00 for two. Of course that's dependent on how HobbyKing is pricing their stuff "today" as their prices fluctuate.
So you have some choices to make. 2 Yuneec batteries for $280.00, and you need three to get the most time in flight, two Zippy's for $120.00, or a single graphene for $118.00. Best flight time will cost you $120.00. There certainly are other batteries out there that will do the job but someone else will have to pick up the tab for that testing