Was just reading about this on FTO UK & also Aussie versions of the conversion on Aussie Magna Club.
Apparently there is a 294mm rotor & with a slightly different mounting bracket that bolts onto the FTO hubs with no modification, and can use the standard GPX calipers.
I'm reading that the 294mm setup comes off AWD Magnas, Ralliart Magnas, series 1 Legnum, Pajero & potentially the Evo 4s.
They don't fit the FTO because the bolt spacing of the caliper carrier for the FTO is 130mm whereas the Magna, Evo IV, Galant etc is 160mm.
There is however a brake caliper carrier that is 130mm and fits the 294mm rotors. It can be bought new I believe but I don't know the part number. Supposedly the very early Galant VR4 had these, and MAYBE some outlander with 294mm brakes.
So yes, once you have these 130mm carriers then all you need is 294mm brake rotors and you can use your twin piston calipers.
Alternatively Evo IV knuckles might be able to be used which already have the 160mm bolt spacing. This also allows for Brembo brake upgrade too