'How Valid is the Concept of Multi-Discipline Competition cars?'

[Rollo]

In theory the bare bones of a modified touring car for Rallying shouldn't be any different to road car modified for rallying.

Those bare bones are only likely to be the unibody, roll cage, the safety gear etc.
Things like springs, shocks, tyres, are going to be totally different. Never mind the fact that the torque curves required for track racing are different to that on loose surfaces.

At some point, you're talking about something akin to The Ship of Theseus.