No, you don't loose nothing. You still have 100Mbps in the RMII (2 data lines x 50MHz = 100MBps) and the control signals are still there as in the MII.
For 1Gbps you need RGMII - 4 DDR data lines and 1 control line plus clock (125MHz). So 4x125=500Gbps but because the data lines are DDR signals you get 2x4x125=1Gbps.
Also, there the serial equivalent for the MII signals, It's the SMII, 1 data line and one 125Mhz clock. It's 125Mhz clock because 2 of the bits are used control. And for 1Gbps you have SGMII with 1 data line and one 1.25GHz clock.
Have a look at the IEEE 802.3 standard for better explanation.