If you look in XUi_Menu/windows.xml and search for XPMult you will find two different entries like this:
<rect tab_key="xuiGameSettingsBasic">
<grid pos="8,-40" rows="15" cell_width="525" cell_height="45" repeat_content="false" arrangement="vertical">
<browsergameoptioninfo name="GameDifficulty" title="goDifficulty" display_names="goDifficulty1,goDifficulty2,goDifficulty3,goDifficulty4,goDifficulty5,goDifficulty6"/>
<browsergameoptioninfo name="DayNightLength" title="goDayLength" value_localization_prefix="goMinute"/>
<browsergameoptioninfo name="DayLightLength" title="goDayLightLength" value_localization_prefix="goDayLightLengthValue"/>
<browsergameoptioninfo name="BloodMoonEnemyCount" title="goBloodMoonEnemyCount" value_localization_prefix="goEnemie"/>
<browsergameoptioninfo name="ZombieMove" title="goZombieMove" display_names="goZMWalk,goZMJog,goZMRun,goZMSprint,goZMNightmare"/>
<browsergameoptioninfo name="ZombieMoveNight" title="goZombieMoveNight" display_names="goZMWalk,goZMJog,goZMRun,goZMSprint,goZMNightmare"/>
<browsergameoptioninfo name="ZombieFeralMove" title="goZombieFeralMove" display_names="goZMWalk,goZMJog,goZMRun,goZMSprint,goZMNightmare"/>
<browsergameoptioninfo name="ZombieBMMove" title="goZombieBMMove" display_names="goZMWalk,goZMJog,goZMRun,goZMSprint,goZMNightmare"/>
<browsergameoptioninfo name="ZombieFeralSense" title="goZombieFeralSense" display_names="goOff,goZMDay,goZMNight,goZMAll"/>
<!-- <browsergameoptioninfo name="PersistentPlayerProfiles" title="goPersistentCharacters" /> -->
<browsergameoptioninfo name="MaxSpawnedZombies" title="goMaxSpawnedZombies"/>
<browsergameoptioninfo name="MaxSpawnedAnimals" title="goMaxSpawnedAnimals"/>
<browsergameoptioninfo name="XPMultiplier" title="goXPMultiplier" value_wrap="false" value_type="int" values="25, 50, 75, 100, 125, 150, 175, 200, 300" display_names="" value_localization_prefix="goXPMultiplierValue"/>
<browsergameoptioninfo name="AirDropFrequency" title="goAirDropFrequency" display_names="goDisabled"/>
<browsergameoptioninfo name="AirDropMarker" title="goMarkAirDrops"/>
</grid>
</rect>
I'd point out the line with XPMultiplier and goXPMultiplier has the available choices. If you want something more than 300%, I think you just need to add it to the list and then pick it when you configure your game.