my set up is as follows:-

9-speed STI lever to XTR rear mech
Avid Speed Dial 7 levers to BB7 MTN cable discs

as far as i can see, the internal and external cable dimensions are the same as road cables… is it the cable ends that differ?

is it the cable ends that differ?

yes, road brakes have a steped cylinder end with the cable exiting axialy, mtb brakes have one big cylinder with the cable exiting radialy.

That and Road bikes usually have different length sections of outer compared to MTB (and definately compared to disk brakes).

Mr Spoon is correct.
.
Gear cables are interchangable though.

Gear cables are interchangable though.

usually, there are exceptions.

Jagwire racer cable sets wont work for mtb. The brake cable outers have a short flex section which you woudnt want to use on an mtb, and im sure the outers would be too short anyway for disc brakes. Best bet for cable discs is BMX linear brake cables, work a treat to add a bit of bite and sharpness to BB7s.