Well Christ I do not believe Diff will continue to rise @ 20%, as of this increase network has fallen on it's face and is running under 6 blocks per hour and has been for almost a day. That 25% increase KILLED the last of the FPGA/GPU miners. Anyone running a BFL single is going to be shutting them down SOON. My 2 60's earn approx $2.04 USD per day, but cost me $1.15 USD so they are on eBay as we chat.
I don't know, seems like 20% is pretty consistent. Average increase since the start of 2014 is 18.15%, and with the glut of new hardware hitting the market, I think 20% may be here to stay.