There's lots of evidence to show that mnemonics boost recollection by a factor or three or so across a wide range of domains, abilities and time ranges. See e.g. http://www.unforgettablelanguages.com/studies.html
Re the intermediate mnemonic device, here's the way I picture things. The mnemonic provides training wheels for your brain, helping you get the answer right a few times. Then, after enough correct responses, mediated by this (hippocampal) mnemonic representation, you rely less and less on the training wheels, and your cortex has had a chance to form a longer-lasting and more direct semantic link.
Disclosure: I'm one of the co-founders of Memrise, so it's not too surprising that I think there's merit in this approach :) Drop me a line or reply here, and I can try and follow up in more detail. Maybe I should write a blog post...