What memory size are your rx480?
4GB
I downloaded the latest version of bitcrack!
Is compiling ....
I think it was the old version that was giving me half of mkey / s
with Rx480 4Gb, this must work better
bc.exe -c -u -d 1 --keyspace 1000000000000000:1FFFFFFFFFFFFFFF -i addrdata.txt -o ohmygodimrich.txt -b 72 -t 256 -p 1024
also watch your core-clock and memory-clock
no need to overclock, you won't gain much. In fact you can lower core clock to 975MHz, you will still get around 100MKeys/s ...