About the max DDR3 rate

Refer to the external memory performance (http://www.altera.co.uk/literature/hb/cyclone-v/cv_52006.pdf) Datasheet, page 2. The hard controllers can operate up to 400MHz. You can rely on these working at that frequency for the expected life of the FPGA.

This also states the maximum frequency of an FPGA operating a soft core memory controller. This is the maximum possible but will be dependant upon you particular design. Quartus will report the max frequency of your design. You can rely on your design operating at that frequency for the life of the FPGA.

Regards,

Alex