[General Questions and Answers] D400:
You can use simply notepad on your pc to edit that file and copy it in the device game directory OR can use any .json editor to edit the values in the device itself.

First of all, make sure that you change the override to "true" from "false". I use settings which are as follows:

{"override":true,"TIER_GLOBAL_HALF_TEXTURES":true,"TIER_GLOBAL_TEXTURE_MAXSIZE":1024,"TIER_GLOBAL_ANISOTROPIC_TARGET_HIGH":4,"TIER_GLOBAL_ANISOTROPIC_TARGET_LOW":4,"TIER_ENVIRONMENT_FOG":true,"TIER_ENVIRONMENT_ROADSPECULAR":true,"TIER_ENVIRONMENT_FULLCORONAS":true,"TIER_ENVIRONMENT_POSTLIGHTS":true,"TIER_ENVIRONMENT_ROADSHADER_DETAIL_LEVEL":4,"TIER_ENVIRONMENT_ROADDETAILMAP":true,"TIER_ENVIRONMENT_REFLECTION_TYPE":2,"TIER_ENVIORNMENT_SHADOWINREFLECTION":true,"TIER_ENVIRONMENT_REFLECTION_RESOLUTION":254,"TIER_ENVIRONMENT_COLLIDABLES_IN_REFLECTION":true,"TIER_ENVIRONMENT_FRESNEL_BRIGHTENING":true,"TIER_ENVIRONMENT_NORMALMAPS":false,"TIER_ENVIRONMENT_SKIDS":true,"TIER_ENVIRONMENT_RAINENABLED":true,"TIER_ENVIRONMENT_MAXDISTANCE":400,"TIER_ENVIRONMENT_LOD0_DISTANCE":100,"TIER_LIGHTING_DYNAMIC":true,"TIER_LIGHTING_SH":true,"TIER_LIGHTING_NITRO":true,"TIER_LIGHTING_NITRO_ENEMY":true,"TIER_LIGHTING_HEADLIGHTS":true,"TIER_CAR_FLAT":false,"TIER_CAR_ENABLEFOG":true,"TIER_CAR_DAMAGE_SCRATCHES":true,"TIER_CAR_NORMALMAPS":false,"TIER_CAR_WHEELBLUR":true,"TIER_CAR_DECAL_HIRES":false,"TIER_CAR_MERGEREARWHEELS":false,"TIER_CAR_LIGHTS_TRAILS":true,"TIER_CAR_CLEARCOAT":false,"TIER_CAR_MAXDISTANCE":100,"TIER_CAR_HIGHEST_LOD":1,"TIER_CAR_SHADER_DETAIL_LEVEL":4,"TIER_SOUND_AIENGINE":true,"TIER_SOUND_REVERBPRESETS":false,"TIER_SOUND_ROADBLOCK_SIRENS":true,"TIER_SOUND_48KHZ_OUTPUT":false,"TIER_EFFECTS_TRAFFIC":true,"TIER_EFFECTS_PARTICLES_HIGHQUALITY":true,"TIER_POSTFX_BUFFERSCALE":100,"TIER_POSTFX_BUFFERSCALE_EVENT":80,"TIER_POSTFX_BUFFERSCALE_ALLOWHALVE":false,"TIER_POSTFX_DYNAMIC_VINYLMAP":true,"TIER_POSTFX_COLORCORRECTION":false,"TIER_POSTFX_HYPERSPACE":true,"TIER_POSTFX_DOF":true,"TIER_POSTFX_DIRTYLENS":true,"TIER_POSTFX_RAINLENS":true,"TIER_HIGHEND_PROJECTED_HEADLIGHT_TEXTURE":true,"TIER_HIGHEND_SUBTRACTIVE_SH_LIGHT":true,"TIER_HIGHEND_DOF_AND_HYPERSPACE":true,"TIER_HIGHEND_POINT_LIGHTS":0,"TIER_HIGHEND_SHADOW_TYPE":0,"TIER_HIGHEND_RAINDROP_SHADER":true,"TIER_HIGHEND_SCREEN_RAIN_REFRACTION":true,"TIER_POSTFX_INTERMEDIARY_SIZE":30,"TIER_OCCLUSION_QUERY_DISABLE":false,"TIER_LENSFLARE_DETAIL_LEVEL":4,"TIER_LENSFLARE_MASTER_SCALE":50,"TIER_COLLIDABLES_DETAIL_LEVEL":4,"TIER_COLLIDABLES_MAXDISTANCE":150,"TIER_SHADER_COMPILE_CLEARCACHE":false,"TIER_USE_BINARY_SHADER_CACHE":false,"TIER_ENVIRONMENT_RELFECTION_SIMPLESHADERLOD":true,"TIER_ENVIRONMENT_COLLIDABLES":true,"TIER_CAR_PER_VERTEX_REFLECTION":true,"TIER_CAR_PER_VERTEX_BRDF":true,"TIER_CAR_EFFECTS_SHADER_QUALITY":4,"TIER_ENVIRONMENT_NO_WINDOW_REFLECTION":false,"TIER_ENVIRONMENT_FAST_WET_ROAD":true,"TIER_POSTFX_FXAA":0,"TIER_ENVIRONMENT_DISABLE_WET_ROAD":false,"TIER_ENVIRONMENT_FAST_SPECULAR":true,"TIER_ENVIRONMENT_VERTEX_FOG":true,"TIER_ENVIRONMENT_FAST_RAIN":true,"TIER_FORCE_1X_UI":false,"TIER_NATIVE_BUFFER_WIDTH_LIMIT":1136}

I have come to this settings after a few hrs of testing the game on a couple of different settings.

If you have a high end and better device than Galaxy s4 then you can even set the "TIER_ENVIRONMENT_REFLECTION_TYPE" values greater than 2(Max value is 4 so don't use more than that).The game lags when i set it greater than 2.

Also, i've kept"TIER_POSTFX_COLORCORRECTION" at "false" because the gpu on my device powervr sgx544mp is not able to handle it even with exynos octacore cpu(extremly slow framerate on setting it to"true").

Keep "TIER_GLOBAL_TEXTURE_MAXSIZE" & "TIER_NATIVE_BUFFER_WIDTH_LIMIT" values to what is already set by default in your game's graphics settings.

MOST IMPORTANTLY the 'TIER_HIGHEND_POINT_LIGHTS" & "TIER_HIGHEND_SHADOW_TYPE" values are set to "0" because the game becomes unresponsive when loading the cars when the values are set at any values between "1" to "4" (happened in my device".

Rest of the options are pretty clear in itslef.Just cross check the options which i've listed specifically so that the game does not suffer extremely slow framerates or freeze.

One more thing to be noted is that same method can be used to lower the values and get excellent framerates(for low end devices).There's absolutely no need of gltools for this game... hope this helps :):)