You could experiment:

1. Lemmiwinks

2. Aux. air intake into the air box. The S6 has that if you ever look at one carefully.

3. Investigate the throttle body. I notice the throttle body is smaller than the intake manifold where it bolts up. I wonder if A8 version with the extra 10HP or the S6 version with 40 extra ponies is actually full size. Kind of like a NASCAR restrictor plate concept. Throttle bodies are expensive for these cars because of drive by wire.

Do realize all the S6 power is really up above the regular 4.2 redline if you look at the power curve for the motor. But most of the S6 real delta is in the tranny gearing--the 4.2 being ridiculously highly geared to avoid the guzzler tax they decided to let apply to the S6.

....

Current #1: '06 D3 A8L W12 brilliant black w/ amaretto & 4 seat setup, GmbH custom, CPO; modestly VAG COM lowered, H&R 20mm rear DRA's with 275/35 on OEM polished + forged 20's; S8 sway bars (see: rear bar and gen'l D3 bar info and front bar); tweaked 385mm brake mod; matching C6 A6 back headrests (better rear view: headrest tweak); owner installed AMI retrofit with part number details here; other VAG COM tweaking to suit
Current #2. C5 '00 A6 4.2 ming blue w/ beige, new off showroom floor; Lemiwinks + VAG COM tweaks; smoked tails; RS 6 OEM 18's; RNS-D (CD) Navi
Prior: '96 C4 A6 2.8Q, green mica; still w/ family; bored throttle body
Prior: '85 C3 5000S, stone grey, 5 sp FWD, 5-3-1 4000Q header, 10:1 Euro pistons (U.S. 8.5:1) w/ turbo sodium valves and OEM ext. oil cooler; VDO guages w/ Autobahn OEM Euro mount; new via gen 1 Audi Euro delivery; ran past 200K
Prior from the primitive "good old days": '73 C1 100LS, grey, bought @ 69K, ran past 200K...even w/inboard solid disks and 165/14's on 4 1/2" steelies...; Weber 32/36 DGAV w/ 9 extra degrees of (CDI) ignition advance

helpful cross reference to a C6 post with MMI and other TSB's that also cover D3 A8's.