'DMS' do up the Audi RS6 to 680bhp or 720bhp... but it's really heavy (& crazy).
Which one has better fuel consumption? I do not know.
Do you intend to even soup up your ride? I dunno abt the Audi, maybe MTM tuning or ATB tuning... I like an Audi or VW to wear BBS 'challenge' wheels, looks nice.
I heard only 2 parallel importer of Nissan GT-R got the Nissan card firmware upgrade, other 'ku ching ku rung' parallel importer just sells you the car & that's it. If you intend to make it even more powerful, it is a dangerous option. HKS has their own firmware upgrade when servicing too, but i am not sure... Or there is Mine's & Amuse Powerhouse upgrades offered for the car... ECU & exhaust manifold sees the GT-R pumping out 550bhp to 600bhp. But your car could maybe manage only 1.9km/litre after that... It is a case of 'bo tai bo chi' the fuel comsumption very high, even if you did not attempt to do 0-100km/h in 3.5sec at the traffic light. Many GT-R used from 2006 cannot sell, standard ones can't really sell, let alone those with HKS & Mine's tuning... they can be had for a bargain, something like $220K onwards. Those with HKS 550bhp option or Mine's 560bhp-650bhp got to be careful... after just a while, the gears cannot handle it, attempting to overhaul the gears & then put it back, the system detects that it has been tempered with & the system shuts itself down dun let you on at all... This is the Nissan anti modification measure, that's why in Japan, when they send for servicing, got firmware update... The gear issue i think the aftermarket firms are still working on it. Aftermarket brakes kits are available from again the previous mentioned tuning house. They are 18" or 19" carbon graphite cross drilled disc with 8 piston & even 10 piston caliper. They cost between $40K-$60K. Another usual option is exhaust system, also cost at least $40K, this one must choose carefully as unlike Lancer Evolution / WRX STi exhaust that cost only a few thousand, you can anyhow change... at $40K+ again, you only want to buy once. Another point is bear in mind those tuned up ones the central tunnel gets so hot that it's almost impossible to sit in the rear. Anyway the rear is too bumpy & uncomfortable even for standard car. Last point, the run flat tyres wears fast & very expensive replacement... I am not sure if got others use lesser expensive tyres... think got but i not sure the diff, not in term of handling but from info feedback from the system... but maybe that is not important as the dash there the info only in japanese wordings, also dunno wat it is saying...
Ok, i really dun know much abt the Audi... less high tech & cheaper to run than the Nissan GT-R i guess.