I have Thinkpad W520 with i7 2720 processor and 8GB RAM (win7 64pro). The RAM is DD3 PC3 either 12800 or 10600 (not sure which) in 2 x 4GB. There are also 2 empty bays.
First check if your motherboard will support 16 GB, if yes, what is the maximum speed it will handle. It seems like it is 1600, but it could be more.
Does it matter if the bays hold an assymetric configuration? Is it faster to have fewer larger blocks?
Yes, it does. How any motherboards out ther have three RAM slots? You need to check if your CPU can support higher than 1600.
If you're lucky, both CPU and Mobo can go higher than 1600. If yes, then get 8x2 DDR3 of whatever that speed is. If no, then find 2x2 of the same brand, same specs. If you can't, then stick to 8x2.
In real world testing, there is no difference between 4x4 and 8x2, but if you get 8x2, you have better chances of using it for the next upgrade.
Universal theory? How's this: Asymmetry in computing is a bottleneck.