A Mk3 rear 'axle' will not give you an extra 1" either side, as I have had both a Mk2 and Mk3 GTi beam (minus spindles) side by side, and it's about 10mm either side. The difference is the thickness in the Mk3 rear spindle.
But, I think hc20v was asking about the rear 'subframe', the one which the steering rack and rear mounts bolt to, as the front subframe is attached to the rad support, and holds the front engine mount. I have a SEAT Toledo version sat around waiting for me to fit, and it's exactly the same as a Mk2 but with the added bonus of a big block of steel to help eliminate torsional vibration.
On my old Mk2 it worked a treat and helped with the massive vibration caused by the poly bushes and Vibratechnics engine mounts, of which I will never fit in a daily ever again!