Nothing that you will even notice. Hard disk cache's are an element of a computer system whose primary aim is to increase overall system performance by saving data in faster banks of RAM.
Hard drives have a cache built in to hold data that is being written to or read from the hard disk. The purpose of doing so tries to reduce the number of occasions in which data has to be physically written and read from the hard disk platter. By retaining recent data within the cache performance is increased as this memory is much faster and saves time on repeat recalls.
The other primary function of a cache is to try to match speeds between devices so that the faster device is not held up, or isn't held up as much. In the case of a printer the cache stores data sent to the printer from the system where it is held until the printer is able to print it off. This enables the system to continue other tasks while the printers cache holds onto the information.
The actual noticeable improvment between the 8mb and 16mb will not be recognizable.
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