In order to see any improvement with the XK parameter the key table need to be reloaded. As for what to expect in terms of improvement depends a lot on your file structure.
The major gain seen when file using XK is that the key blocks, the pages of the file used to maintain the keys, will be able to hold more than 255 keys. For example if you have a file with a 8 character key, each key entry in the key block becomes 13 bytes long so the maximum amount of space in a key block used will be 13 * 255 = 3315 regardless of the block size. So if you are using the default of a 4K block or about 20% of each block cannot be used. Enabling XK allows the system to put more keys in a key block (~ 64,000) resulting in smaller file sizes and faster key access due to a shorter key tree.
Now if your keys are such that 255 keys will always fill a key block this will make little difference, however if your file has say a 16K page size, any key less than 60 bytes would result in wasted space.
Also, the XK parameter allows the system to create VLR files beyond 2GB -- so if you need larger files you may want this parameter enabled.