The 6GB configuration produced essentially identical benchmark results to the 4GB configuration in our 2.8GHz MacBook Pro. The difference ranged from -.5% to +1.7% or an average difference of +.28% or less than 1/3 of one percent. I call that negligible. Or, in other words, you should have no worries about a speed penalty imposed by non-matching memory modules and loss of interleaving.
And when you consider the 6GB configuration potentially means less virtual memory swaps, then the only consideration is the cost.