I had the same problem , i was doing same thing as you . Farming core with a barb , i was around 220k per hour.But then i bot lvled a DH on my bot account.I saved all my gold so when my DH was 60 i could start farming sarkoth.Now i have a lvl 60 DH with 280 % GF , 24 % MS. Im doing around 420-450k per hour.
In my opinion , its good to start with core but if you can you should farm sarkoth inferno.there are alot of guides and profiles for sarkoth , you just have to try all of them and see what is the best for you . I dont really care about getting max GPH , i just want stable and decent amount of GPH.