The memory controller on Intes current cpu's is very good, and is able to keep the cpu fed from any speed ram.
I think ou will see very little difference in dual channel operation vs. quad channel operation for most tasks.
Such benchmarks that are not synthetic, but represent real apps are hard to come by.
For some insight, you might read this on the effect of ram speed: http://www.anandtech.com/show/4503/sandy-bridge-memory-scaling-choosing-the-best-ddr3
For your plan of buying a second kit, there could be some issues.
1. Ram is sold in kits for a reason.
Ram from the same vendor and part number can be made up of differing manufacturing components over time.
Some motherboards can be very sensitive to this.
That is why ram vendors will not support ram that is not bought in one kit.
Although, I think the problem has lessened with the newer Intel chipsets. Still,
it is safer to get what you need in one kit.
2. A motherboard needs to supply the same voltage to all ram. It is easier to do this with 2 sticks compared to 4.
If 8gb is really all you need, then I would just keep the kit you have.
If you want 16gb, go ahead and buy a duplicate 8gb kit. I think the odds are remote that you will have a problem, but no guarantees.