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.

ethash is similar

.
Of course, I didn't want to imply that its NOT important, just not as important as with ethash, where it almost doubles performance...here
you still gain 30-40%, but with ethash its 70-80%, and up to 100% on my insanely clocked RX560 Gigabyte
Added power consumption to first post