firstly i think bugger the auto!
but if u want one, here's what i think.
if u want more torque go with an A14 or 15 as they bolt right up to anythin the A12 does.
but if you want an A12 then perhaps an over bore with higher comp piston and thinner head gasket would be ok.
if you want to bore it, dont go any more than 2mm. if you do there is a possibility of warping the bore because of insufficient metal for support under normal operating temps.
ucan do the maths. if u bore the standard A12 bore dia 2mm more, u'll end up with 75mm. volume of a cylinder = 2piR^2 x stroke x 4cyl. (two pi radius squared multiply by stroke multiply by four cylinders)
i hope thats right. if u bore it 2mm u should have about the capacity of an A14 or 15. but then you need custom pistons to suit and all the other mucking around.
i would buy an A15 and be done with it!