You need to set the processing template to None or function that does not include DRA.
'None' will return you the Radiance values (currently scaled 5000 - 55000, but will be changing soon to be scale from 0 - 10000). If you set one of the other processing chains then it will be returned stretched to 0-255.
If you use None then things will slow down as the server will be returning 16bit data. You can speed this up by setting the compression to LERC and using a tolerance such as 4 or 8.
Optimum is to create a function and transmit it to the Server to be processed there an return only the result. This can be done by using the Raster Functions in Pro.
If using Desktop also check out Landsat 8 Image Service Add-In Tools
http://www.arcgis.com/home/item.html?id=f331af8569fb4f19bd74954eddc44bd3