On my 290 memory controller load is rarely over 60%. Big difference is that beside 290 memory bus being 2x wider, memory runs at 1250 MHz vs 2000 MHz on your 480. This means that you can do all possible tricks but no way can use that tight timings as on 290 at 1250 MHz or 390 at 1500 MHz. OK suppose that you reduce mem clock on 480 to 1500 or 1250 MHz to get the same timings but then you still do not get the speed that is possible with 2x wider bus.
This is not question of memory througput. Reducing memclock almost twice affects only to 20% hashrate drop.
Update: Tahiti have 768 kB L2, Hawaii have 1 MB L2, Ellesmere have 2 MB L2 cache.