What is the best cluster size for a ReadyBoost ntfs memory card?

I started testing cluster size and I found that for my 8Gb thumb drive the best format was Exfat at 4096Kb. I tested with DiskMark. Speedgain at 8192Kb was negligible but there was gain over 2048Kb cluster size.

Sequential Read (Q= 32,T= 1) : 19.295 MB/s Sequential Write (Q= 32,T= 1) : 7.864 MB/s Random Read 4KiB (Q= 32,T= 1) : 5.673 MB/s [ 1385.0 IOPS] Random Write 4KiB (Q= 32,T= 1) : 0.015 MB/s [ 3.7 IOPS] Sequential Read (T= 1) : 19.084 MB/s Sequential Write (T= 1) : 7.968 MB/s Random Read 4KiB (Q= 1,T= 1) : 5.321 MB/s [ 1299.1 IOPS] Random Write 4KiB (Q= 1,T= 1) : 0.010 MB/s