I didn't play with cryptonight too much lately...but I think memory frequency is less important than core frequency for this algo...or, at least,
much more important than for ethash
Sorry, but the memory frequency (the memory speed) is very important for cryptonight. The main loop is iterated 524,288 times with random read/write/aes/xor operations to the 2MB scratchpad. much more important than for ethash
ethash is similar
.