Yes we know. You need ollyDBG about that.
Open sro_client.exe with ollyDBG
Ctrl + G > 006A51BC
You should see something like this
168 is your mastery limit in HEX (It could be different number in your client.)
Double click on it and change it to => MOV EDI,0x1E0
Now again Ctrl +G > 006AA4C3
You will see a line like the one above
Modify it again > MOV ESI, 0x1E0
Right click somewhere > Copy to executable > All modified selections (or something like that) A new window will pop up, right click again and click save file to save the changes.
If you don't understand that, quit making a server. Have fun! (It must be modified server side as well but I assume you are using any of the released GameServers and you just specify in the *.inis the mastery limit.)