Double thumb is awesome for Metal.
Add slap+ metal, that's not bad either, better than fatty 1/8 notes, although that was Hair metal 110%.
I tried some old Maiden tunes with Double thumbing, it was Fun!
I switched over to some punk and thrash tunes, it's a lot easier and sounds really neat.
It takes awhile to get it, but it's well worth the speed gained.
It also helps to play a more chordal approach, at least for me.
Scales can be ripped off pretty quick, funk additions, metal scale renditions of funk technique, a wide range of sounds can be had and of course, you can finally out do your guitarist that has that over size ego (Every metal band has one of those).
Now work some tap in with that.
See M Manring, and Wooten and add some of scales and ideas you have about metal into the fray.
But sit down and force yourself to learn the methods/techniques and they will come, just keep after it. I recently learn it, and I cannot say how much I am pleased.
If I can, so can you. See You tube for instructions.
Another
Add some Punk drums, some metal guitar, and that's a fun time.
Regards,
Tom