I have no mods yet - it is away at an Audi bodyshop just now having a whole new tailgate fitted as the original one was rusting from the inside (I sneakily think they repaired it from a bump and it didn't take).
I kind of thought having it in standard form would be better that grill blocks etc. for this event as I'm still in shock that they repaired it under warranty - +1 for Audi, top people.
On the other hand I agree that Audi parts are very spendy, as is the level of work required if anything does go wrong - timing chains for example means engine out (they are at the back of the engine bay) and about two thousand quid
but luckily mine has had the mod to stop timing chain noise and excess wear / stretch.
Mine is a conventional Auto (no DSG, no Multi-Chronic) which is the main sapper of MPG for me as I drive mostly in town. On a longer run I can get it into the mid or late 40s (imp) but as I posted the real MPG is lower by about 10-20%. At 70 in top the engine is doing under 2.5K RPM.
Around town (day to day) I get about 30-35 every day which is below "EPA" (ECE here) but I like to compare my record against the ECE "Urban" figure which is 27 MPG - mainly so I feel better...
I do have plans to start taking out and examining pipes in the induction system and check for carbon build up and if the swirl flaps are working - there is an updated kit available. I also plan a plug in tuning box rather than a remap so I can go back to standard if / when needed. I also plan an EGR delete which a few UK based TDI V6 owners have done to some benefit in terms of smooth running and MPG.
Being honest though given it is a 140 MPH Auto AWD machine, getting mid-30s out if it around town seems pretty decent.
I look forward to your aero mods.