Yeah, the spring rates I'm throwing around aren't 100% accurate. But I'd be surprised if they were worse than 95% accurate. And like you say, the rates are not constant... I'll be very interested to see how you get on with the D8 setup, especially at the front. Please get good before/after ride height measurements.
That thread you linked was about Xd stuff...what was the takeaway from it? Sorry, I read through, but didn't come across anything I didn't already know (ish).
Regarding your existing setup, I completely agree with what you said about the RSFB - made everything so squidgey that it was impossible to tell what was what back there!
Regarding the spring rates, I think upping to ~145lb/in won't present a problem in itself - you'll still have a much lower front ride freq than rear (softer front (compared with rear), as is the norm on passenger cars). And I think the front stiffness will still be around the 'sporty passenger car' realm, rather than track/race car.
I think with those additions in your front end, you'll end up with 15~20mm increase in front ride height. I could well be wrong though! And if that is the case, you can just swap back to your stock spring perches and you'll be back at stock ride height if desired.
What bump stops will you go with? I would/will switch to something much shorter if/when I do mine.
I think there's another option too...M3 front springs! Obviously the M3 strut mount is way different from a non-M one, but the spring perch rubber on the strut itself is the same part number for M and non-M: 31336764372. I think that should mean that the springs will fit our struts, with the possible exception of the top spring perch. With a suitable sleeve (M14 vs M12 shock threads), the M3 top mount could fit in our cars, but I have no idea about its stack height. M3 shocks don't fit our cars without the steering knuckle (and the lower arm for the rear)...never mind the EDC stuff.
M3 springs (could) give the option to have much firmer front end - up to about 1.5Hz ride frequency, which could match the rear end nicely...I'm working on numbers and things in the background here, but have reached a point where I need to know the front and rear roll centres, and CoG height. I may have to resort to faffing about with a tape measure on my car, and drawing the suspension geometry up in CAD...accuracy would be debatable, most likely, but should be better than a finger in the air.
Also been looking for data/specs on Alpina springs, but can't find much; I imagine they'd be a more sensible middle-ground between non-M and M items. All I've found on them so far is that they're likely manufactured by Eibach, and they have the Alpina part no 31 33 130, which seems not to exist outside dealerships. Alpina parts catalogue:
https://drive.google.com/file/d/14UT...ew?usp=sharing
Come across a searchable version of the Suplex spring catalogue, which has pretty handy notes on spring nomenclature:
https://drive.google.com/file/d/1eEW...ew?usp=sharing
Lists X and Y springs (M3), as well as L springs for Xd cars........
I've been using Autodoc.co.uk to find specs on each spring (dia, wire dia, free length).