This is a weird one to set. It seems that the dvar scr_tdm_score_kill doesn't exist anymore, which means no more High XP lobbies. But I found something that will allow you to create a 4x XP lobby.
Exemple: 400 XP/Kill in a TDM Match instead of 100
How To Set It:
1. Read String value (13 Characters) of 0x05A42310. You will get "scr_xpscale 1"
2. Change it to "scr_xpscale 4"
3. Start Game as Host
Warning! Setting scr_xpscale higher than 4 or lower than 1 will cause the match to crash and bring you back to the lobby. "scr_xpscale" dvar affects all players connected to your lobby.
Also, If you have Double XP activated in your profile, I think you will have 8x XP.