Core2s have significantly better memory bandwidth, because they're dual-channel. They stream memory significantly more efficiently than N2800 Atoms. When a Core2 Duo has a memory stall, it can start executing other instructions out of order to compensate. Typically, out-of-order cores seem to do more for efficiency than hyperthreading.
If Apple screwed up by only giving a single DIMM (it happens on lower-end hardware, but I'd be pissed if I learned that an expensive Macbook were improperly configured memory-wise)... maybe the Atom would have a chance. But a properly-configured dual-channel RAM Core2 Duo, as old as it is, still would stream data faster from RAM than an Atom could. Almost twice as fast in fact.
If the N2800 does perform better on this workload, I'd be definitely interested in seeing the exact numbers... if you don't mind.
I actually do own a Hyperthreaded Atom (N570) and Core2 Duo (some old crap desktop version) myself. So... I'd be able to repeat the experiment :-p