nr_requests doesn't affect the amount of data cached before writing, or the duration before data gets written. MMC/SD and usb flash drives behaves completely different than real SSDs. The emmc in n900 will do something like 8megabytes/second writing sequentially, and 30 kilobytes/second when fed random writes. That means the better sorted requests to it are, the better throughput will be. Note I said throughput, not interactivity, which is probably what users mostly notice.