as i told before, my home network is fully gigabit compatible. I have a Dell 10 Gigabit switch, my router is also gigabit capable. All of my hardware is set to jumbo frames. I run 200m of fiber in my home, it's a whole block 
When i asked the question i forgot to tell that i have a computers science degree, i'm a network engineer and have a lot of knowledge on the field. Instead of telling that the default 1500 will do just fine, you could have just answered the question with useful information, because i know what i'm asking about. If this was a real linux distro, it would only need information written on the interfaces file, but as libreelec and also openelec reads it's network configuration from a xml file written by LE configurator and LE configurator doesn't have a MTU option, it would make sense to mess with the xml.