It sounds like you've made your mind up so get the GXP - but like I mentioned will be MUCH harder to find one w/ the right combo of price/color/options/proximity.
Everyone's position is different. I personally wanted to get a GXP, but ONLY due to the manual tranny (the SOTP feel between A6 gxp and A6 gt is not significant IMO) but at the time they were literally just hitting the lots and dealers were not budging in price much at all. So I got a new leftover 2008 for a good deal ($28,000) w/ all the options.
Another thing to consider is how much you drive - at the time I purchased and really only up until recently I was putting on an average of 23,000 miles a year, so while I immediately put on an intake/exhaust/tune (which nearly equaled the stock A6 GXP power-wise), the other differences that I upgraded like brakes/suspension took care of themselves through routine maintenance since I was racking up the miles quickly.
W/ the way the prices are now and in my current situation if I was shopping for used G8 it'd be the M6 GXP and only the M6 because there is nothing else that compares (4000 lb V8 RWD 4 door w/ stick) save an M5 and that is $90,000 new and much more than a GXP used.
IMHO opinion for the reasons I stated the A6 was not worth the premium at all TO ME - to others may be different. Good luck w/ whatever you get either way you will not be disappointed.
'08 GT sport/premium, tints 20/35, 18" TSW nogaros w/ bridgestone blizzak winter, 19" sport oem w/ hankook evo summer, Philips crystalvision bulbs, debadged, Vararam OTR, JBA 1-3/4" LT's, ported TB, MF axlebacks, MF X-pipe, DBA-powerslot brakes, russel SS lines, hawk ceramic pads, GXP shocks/struts, lovells poly strut mounts, Slowhawk Tuned 344/356
Gone -'04 GTO A4 2.5" custom exhaust, sweet thunder muffs, H pipe, Z06 heads, Baer brake it, 20% tints, torrid red, AEM brute force intake, tuned