OEM or Aftermarket Springs? That is the question

Great that you're having fun with the car! How are you trying to make the car more fun to drive? Tell us exactly what you don't like and what you want to change - figuring that out will help you with what needs improving.

As for the smaller wheels, I would stick with your 17's or move to an 18" package (the stock 19's always seemed to be a bit overkill on the weight/sidewall compromise). If you were racing lightweight 16's would be better, but regular driving on real roads will reward shorter, stiffer sidewalls (better response). Adding sidewall adds compliance to the system and can dull out response and feel, everything else staying the same.

If your local weather permits it, go with a max performance summer tire (Pilot Sport 4S is my current go to) - you will gain sidewall stiffness and tire grip over an all-season. If you have a little bit of snow or just sustained cold weather (below 45F) and don't want to have summer and winter tire sets, look at high-performance all-seasons like the Michelin Pilot Sport AS4 or Continental DWS06. The difference between those and the two you listed is quite large for grip and performance driving.

I'd probably stick with your stock springs; I've dealt with the scraping on a mild drop on my old 1st gen 6, and my stock height ATS and it sucks to deal with. I could dive into sway bar changes and chassis braces, but you really need to define what you want and the compromises you will make to get there.

Looks like you're being very thorough, that will be a cool build! What strut bar (F & R) are/have you done? As for springs there might be some out there with a mild drop but increased rate, otherwise you could look at coilovers down the road. I'd wait to queue up any suspension stuff past your scheduled the rear shock replacement; best to see how you like it and if you want to change it more before compromising the ride and NVH too much. If you need to scratch the itch though, there are a number of chassis braces you could throw on the car - I think Ultra Racing is still selling a number of them Ultra Racing Mazda 6. Find the part numbers and search around for best pricing/delivery.

As for the tires, if you don't really get snow you could get away with summer Pilot Sport 4S even if it gets below freezing a few days a year - I often end up running my summer tires in Michigan into freezing temperatures. Otherwise the Pilot Sport AS 4 would be a good match on your 17's if you still want some all-season-ability. I think you're going to be very surprised at what a difference those will make.

As for turbo/super, @WoodinvilleDan supercharged his car and it sounds like an absolute riot, plus it seems pretty straightforward on the install and integration.

You could always add a big brake kit to your dream list! I love the 4-piston Brembo's on my ATS; if I ever go back to a Mazda I'd definitely be looking at a multi-piston big brake kit right away.