I have an Orange ST 4, the rear mech cable is in three sections, This causes problems with grime jamming up the cable quite often, especially the section under the BB.. Can I just drill out the mounts on the frame and run a full outer from front to back ?

ta.

I don't know why people prefer to drill out the mount (potential danger of going through the frame etc. as drill can't be properly lined up) rather than just get a hacksaw and cut off the closed end of the guide.

or ziptie it onto the side to see if the shifting isnt slow and just as annoying?

Maybe try these.
It fitted a set as I was having the same problem. They come with a rain cover boot which you can put over the exposed cables. Seems to have cured the problem for me.

>Can I just drill out the mounts on the frame and run a full outer from front to back ? <

Yup – or do what Big John does with his (allegedly) superior technique 😉

I drilled mine (carefully!) but you could also file them if you're worried or hacksaw as above.