Brother what I lack in google research skills I more than make up for it in practical experience regarding actually having test driven these vehicles. Audi may have provided the same (1.8) trims in other markets but fact of the matter is when you sit in a mid-size sedan like the A6 and the sales person tells you it comes with a glorious 1.8L engine then all you truly feel is egg on face not the engine revving. Sit in one with a 3.0 Quattro and you will truly feel like you are getting your money's worth.
Lighter engines work but they only work in sporty vehicles with lighter frames. Case in point being the new Toyota FRS with a subaru plant in it, even at 2L that thing is super fun to drive why ? because its a light ride. A6 is not, thus 1.8tfsi is inadequate especially when you are paying the price that would fetch you 3.0 in any other country.
Despite all this if you are telling me you are happy to pay 70 lac or whatever it costs to get an A6 (1.8) then I wish you good luck and safe driving, as for me ? I'd sooner get robbed at gun point then pay for a underpowered dud and I don't care if it has Audi written on it.