Well, if you are ok with the life/wear and bite of the orange stuff then there is no reason to change. My problem with them was that they wore out very quickly and took forever to bed in as the rotors got worn. This affected the fronts more than the rears, so I would end up having a brake bias that was a bit too much towards the rear over the majority of the pad/rotors life span.
I would probably start with the DTC60 front and rear if I had the choice. Preferably on new or good condition rotors. Then I would do some sessions, or at least two with plenty of cool down between them (ie 2 hours or so). Look at the brake data from each session (I use DME connected AiM data logger). Does ABS activate both front and rear during heavy braking? If you dont have datalogging, you can use a custom configuration of MHD to log the wheel speeds. Problem is the frequency probably wont be high enough to see whats going on. In the end you can also just try to feel what the car does under braking using the butt dyno or by looking at video recordings. You also need to evaluate how the car behaves under trail braking. Does it tend to push or slide? Is it easy to modulate trail braking with the brake pedal or is the bite very sensitive? If it pushes, or if the front abs activates much more than the rear, I would swap to DTC70 rear. If the rear ABS is working much harder than the front ABS, and the car wants to slide when trail braking, and the brake pedal isnt too sensitive, then I would swap to DTC70 front. If the brake pedal feels too sensitive and hard to modulate, then go DTC50 rear instead of DTC70 front.
I have been running H&R bars myself, but ended up modifying the front bar to make it adjustable, and had the rear bar custom made to be a bit stiffer. I like the stiffest setting front and had to stiffen the rear to kill some of the under-steer on my car. This in turn has made corner exits more of a handful, but overall the car is faster when balanced towards over-steer.
As for springs I just run oem front and H&R rear with a "spring spacer" to bring the rear back up to the stock ride height. I dont think this is ideal for track use (hello bump stops), but it makes the car somewhat acceptable for use on the street and further helps to kill under-steer. I have never tried linear springs, but I am sure they are wonderful on track.
As for the original topic - flex - I would look into other tires first. I cannot think of a softer track tire than the cup2s. The grip is amazing in the beginning, but I am pretty sure that any other track tire you might select will cause less flex in comparison... be it trofeo r, ventus td, or pretty much any semi or full slick tire. The ps4 tires are even softer, and of course overheat after one lap on the track as they are meant to work well on the street, in the rain etc. Personally I swap wheels and brake pads at the track and use semi-slicks or full slicks in the right sizes. The tires transform how connected the car feels, and it amazes me every time. Depending on the compound, semi slicks (like Hankook TDs or Z214s) might only make you a second or two faster compared to a brand new set of cup2s, but they will feel so so much better to drive on track. Unfortunately you cant use them on the street as they would wear out in no time
As long as you are using rubber in your suspension arm bushings, dont worry too much about chassis or subframe flex. Do the easy stuff (like the bolt on bars) if you are bored, but know that these are heavy street cars and most or the flex will be happening in the suspension and tires anyway. Just look at the rear tires in this random picture.