So many variables in the decision process. Depending on what type of car(s) you're coming from, and what you're hoping for in your next car...the decision could go either way. I purchased my 2016 S6 last spring after spending a couple of years in a 2013 BMW X5M that was Dinan stage 1. I wanted something still fast, but less aggressive/loud and with far better fuel economy. I tried to convince myself to like the A7 (need a rear bench so couldn't consider the S7 without biting off a seat swap) and the A6 in 3.0T prestige form, even drove a Nardo A6 with the sport seats/red stitching (can't remember the name for that setup) and although quick, the guttural "oomph" I was used to in the X5M just wasn't there, and at this stage in my life I get 1 car to do everything, and it has to hold 5 people. I probably looked at A7s and A6s for 5-6 months, debating if they would fit the bill for me, and considered tuning, etc. Then, one day on another trip to the local Audi dealer to check out a new A7 they had come in, I saw a lease return that hadn't even hit their lot yet - and ended up buying it.
Nothing really replaces the V8TT setup if you move down into the V6s. But, if you haven't been used to "more" on the road, the driving experience in an A7 or A6 is likely more than enough. I had fun driving all of the cars I tested, for me I just put having the V8TT above all else.
As for buying a car with 14 months left on warranty but no CPO - I did that with my X5M. Prior to it coming off warranty, I had a trusted BMW specialist shop do a full inspection and then took that report to BMW and had all issues, even little things, taken care of under warranty. I never bought an extended warranty, but did offload the M with about 45k miles on it as I could feel the tide of repair bills looming. Lots can go wrong with hoses, seals, etc. when you jam a high HP twin turbo V8 under the hood - lots of heat generated so things will just eventually fail over time.
Happy shopping...I actually think I enjoy that more than car ownership.
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