Half of what you listed as Solid Rear Axle Mods also apply to IRS.
I own a 2013 S197 V8 so I know what its like. I've owned it since new (pre-ordered). I also have a 2001 Firebird V8 (owned since new). Both are Solid Rear Axle. The S197 is more comfortable than the f-body, but the f-body gives a raw-er/sportier feel. Adding a Watts-Link to the f-body will improve its track performance. I know its not the same comparison as the S197 vs S550.
When I drove the V6 and Turbo4 Mustangs they were extended rentals (months, not days) in California (I travel for work and often have to stay in an area for a while... but not in the past year). Where I stayed, the average street limits where 35-50 mph (not the typical New England 25-35mph). The V6 rentals were 2010-2011. The Turbo4 were 2016-2019. Not everyone is driving full-tilt all the time (and you really shouldn't on populated public roads) . Plus my cars and the rentals were autos. I did drive some twisties but I'm not a racer.
Having V6 Mustang rentals in 2010-2011 is what led me to buy my 2013 (ordered in late 2011 and picked-up may 2012). I compared the V6 and V8 and ordered the V8. However, if I was in the market now for another Mustang, I'd really consider the Turbo4 with Mag-Ride suspension. However, I like variety so the next car to add will probably be a sport sedan.
The suspension on my Mustang is stock although I have springs, sways, shocks, control arms in box waiting to install.
The suspension on my Firebird is not stock, shocks, sways, control arms, etc. I'm running roto-joints not rubber or poly.
I'm not saying the S197 is bad. Its not. Its good. Its just that the s550 is better.
The V8 will always give you more power. If you never owned one and are interested, then go for it. If you've had one in the past and want something of comparable power to the older V8 (the newer one has more power) but better fuel economy, then consider the Turbo4 or V6 if you really want to avoid a turbo (just stick with the later V6 and not the earlier one).
2001 Black Firebird Formula 5.7L V8 --- 75th Anniversary Package
2013 Blue Mustang GT 5.0 V8 Premium