I wouldn't say there are any reasons not to go for the V8 other than your choices will be more limited (but there are still a good number out there). If I had gone for a V8 I would have needed to travel several states away to get one. I get terrible gas mileage with the T6 so efficiency wouldn't really be a consideration for me and the V8 is rated similarly to the T6.
Let's see...aside from the engine there are styling differences between the two so that should go into your consideration. I strongly prefer the wheels on my T6 to any of the wheels I've seen on V8 models and I also strongly prefer the interior of mine as well. V8s, as far as I know, all came with the shiny classic wood instead of the modern wood inlays and the vast majority of V8 models also have the wood down the center console. The only way I would have dealt with the classic wood is if I had an executive model because at least I'd have better seats, floormats and interior lighting. The V8 models also had the exterior chrome accents before the rest of the line got them (I've added them to the front of my T6). Also, think about your circumstances and what not before you choose an interior color. I think the lighter color looks fantastic with the matte wood but it's very susceptible to getting dirty. The shifter, steering wheel, turn signal stalk and seats will all pick up dirt and start to discolor. I would not have been able to have the lighter interior because I wear raw denim and would have ended up with an indigo stained seat.
There hasn't been much to worry about reliability wise. If you scroll back through the forum you won't find many people with issues. The two issues I've had with my car were both present when I purchased it and they were fixed for free under warranty. The hood latch malfunctioned and thought the hood was open at random times. It was annoying while driving because a message would pop up all the time but it was even more annoying when parked because it would trigger the alarm sometimes. The keyless door handles also didn't work on my passenger side but I don't think that's common. My windshield washer nozzles and headlight nozzles also stopped working one day during a very messy trip I was taking for work. I had to stop at every exit and clean my windshield by hand and it was terrifying at times with all of the snow, dirt and grime building up on my windshield. I don't know what caused it but the system seemed frozen despite it being heated. Never happened again after that but the winter was pretty mild.
As far as necessary features I absolutely would never purchase one that didn't have PCC but I'd imagine most if not all V8s you look at will have it. Being able to keep the key in my pocket at all times is something I've gotten very used to and can't imagine owning a car without it. The heartbeat sensor is pretty useless though.
Many people enjoy having the xenons because they bend as you drive around corners. They're the one thing I really wish I had on my car because it felt like a downgrade from my last car to lose xenons and after being used to a certain color light driving at night made me irritable. I eventually put in silverstar ultras to make it a little better. Depending on what year you go for getting xenons requires getting the sport package which will give you ventilated seats as well and the active suspension (4C). I've heard that improves handling but can also lead to increased tire wear and possibly cupping or something.
Beyond that...many people seem to find the blind spot warning system (BLIS) annoying because it will give a lot of false positives at night and in the rain. Driving through traffic will have both lights flickering on and off pretty often. Not many people have talked about radar cruise control but after xenons that's the next option I wish I had (I only have the climate package which is mostly a result of the lack of inventory available around me when I purchased my car). I do remember some complaints about the cruise control being used on two lane highways that have turning lanes. If you approach a car waiting in a center turning lane on a highway where the lane that isn't turning is slightly to the right (hard to explain but the lane would shift right at every intersection to accommodate the left turn lane) the car will slam on the brakes sometimes. If I had that problem often I feel like I would stop using that feature. The navigation system also isn't that great, especially because it isn't a touchscreen and requires you to operate it with a remote or buttons that are behind the steering wheel. The Dynaudio package is supposed to be very good though and doesn't seem very common so if you find one with it that's probably a good find.
Overall my car has been a dream to own minus the disappointing gas mileage. I'm averaging down in the 16s right now with mixed driving but absolutely zero traffic and few stop lights. I can't get anywhere near 300 miles on a tank before the fuel light comes on. I can't really comment on aftermarket stuff because everything I've done to my car has been oem. I have a 2008 and I added the 2009+ style rear emblem with the spaced out letters, the 2010+ front grille with the larger logo and added the chrome details to the front from the executive package and V8 models. The only non oem thing I've done is added the silverstar ultra bulbs to my headlights and changed my parking lights to LEDs in the front.
If possible, I would go for a certified model just for the added peace of mind. If you live somewhere with lots of inventory or have the possibility to travel to get one make sure you spend time looking around. The prices of the S80 can vary widely.
