Matched RAM on Intel Macs - Mac Guides
As I said, marginal improvement but it's there in some cases. More RAM vs. Less matched RAM = More RAM wins. But all things equal, matched pairs are better.
To quote from Bare Feats'
Real World Speed Tests for Performance Minded Mac Users
"April 20th, 2006 -- Matched Memory Pairs make your Intel Mac Faster. Some of you were asking whether you would gain any speed using matched memory pairs on your Macbook Pro, Intel Mac or Intel mini. In our testing, iMovie renders were 3% faster with matched pairs. Our Photoshop CS2 MP actions ran 6% faster with matched pairs."