I think that generally a computer will work at the speed of the slowest ram. More expensive top quality does not necessarily correspond to better performance, but can give better reliability. Cheaper ram can use thinner gold plating on the contacts, or less stringent quality control of the chips or assembly. Some motherboards work better with certain configurations of memory size in the banks. The only way to really find out is by experiment, but the difference is likely to be so small that you would need an accurate and repeatable method of measurement.