You don't need a flex fuel kit per se but it will allow you to use a mix of e85 and 91/93 pump gas in conjunction with flash tune that accounts for ethanol fuel.
TL;DR: You need a flash tune to run anything above e25. It is recommend you get a flex fuel kit.
I have the PowerNeedy catted downpipe and didn't encounter any CEL with my stock tune or my Titan ProTune piggyback. I have since moved to an ECUTEK flash tune and no issues still.
I would recommend getting it either a cerakote or swaintech white lightning coated prior to install as it will...
There's cars and coffee events that I'm sure any Supra owner will let you take a look at. The C7 Z06 is a silly fast car in it's own right and stock vs stock, the Corvette is way faster. My roommate owns a C7 Stingray and there are a lot of things I like about it and a lot of things I dislike...
There are only two options for full replacements that I've found so far.
Seibon (https://seiboncarbon.com/products/rd20tysup.html)
Fly1 Motorsports (https://www.fly1motorsports.com/2020-toyota-supra-gr?category=Rear+Diffuser)
Thanks. I have an MST intake and a Verus heatshield.
The rears are cake to do. The fronts are a bit of a pain. You have to watch out for the height sensor (just disconnect and remove it prior). Also, you'll have to install it the ghetto way (slide the spring out from the top perch) unless you...
The ride is firmer than stock but still comfortable. Made it out of the city in one piece back down to VA. I have it adjusted with the perches the way they came from HKS.