it'd be a little hard for me to speculate as to exact spring rates & suitability of each as I haven't completed & fully tested my coil overs yet, engine came out before that ever happened, but I can still give some input...
first of all, the reason the rear rate is higher than the front is because of the different ratios of leverage placed upon each, from general observance it seems that the rear require aprox 25% stiffer than the front according to what the aftermarket are spec'ing, BUT, from my own experience I have found that the handling will improve even more by raising the rear rate to above 25% over the front, in some cases well above. I have found that increasing the rear spring rate really improves the fx's horrible tendency to "walk sideways" in the bumpy stuff in turns in the rear, so I guess I'd keep that in mind. my handling went for good to great by just increasing my rear spring rate by ~40% over what it was originally
another thing to keep in mind is that increasing the spring rate itself will help performance, but sacrifice comfort as the trade off... well there is another part to the equation & that is sway bars... I think if the sway bar rate could be increased INSTEAD OF the spring rate, this would also lend similar benefits to handling without as severely impacting comfort as much as raising the spring rates will. the fx is heavy & a higher center of gravity, increasing stiffness goes a long way towards reducing body roll, but body roll could also be kept in check with increasing the anti sway bars & if I was looking to do an "ideal" street setup I'd def look heavily into increasing & even providing adjust ability of the sway bars... adding ~25% stiffness to the rear sway bar of the fx alone would result in much improved handling without sacrificing too much comfort all other things being equal...
as far as your pillow ball vs rubber mounts question, again, this is a direct trade off of comfort vs performance, pillow balls will not deflect & will not absorb, increasing the direct link of dampener to movement which does increase there performance & help maintain suspension geometry in theory, but the flip side is this direct link is also going to add harshness & at least some noise to the ride... becomes a matter of priority... personally I would rather not trade off comfort for only marginal & unmeasurable gains on a street car, so I built my coilovers with the stock rubber mounts.... stuff like pillow balls, solid motor & tranny & dif mounts, etc, are all cool on paper but in the real world to me anyway, the trade off is not worth the insignificant improvement, your giving up too much for too little to gain & in the end some guys that overdue what they see all the racer dudes doing, wind up ruining there ride quality for no good reason... to me if your looking for the benefit a pillow ball is going to have over rubber mounts then you should also look to replace your suspension bushings, as they will also deflect quite a bit, more so even that the strut mount... to me I wouldn't consider pillow ball mount being a benefit or even necessary unless the control arm bushings have already been addressed, but at that point were looking at a harsh ride.
far as your mono vs inverted vs external mono, obviously the "best" would be an external reservoir, inverted mono tube, but, were talking exotic here, the benefit of reduced unsprung weight may be hard to justify vs the extreme costs involved in such a setup, on a street car I'd say that a quality shock is the primary concern... quality as in longevity... any company can produce a shock that performs out of the box, but a race car may do less than 1k miles a year & then service the suspension, where a street car needs to have shocks that will last for 30-40,50k miles... for that I'd look to the leaders if at all possible, think KYB, Koni, Bilstien, etc... I'd say that the longevity of dampening performance will be the big difference between an unknown china manufacturer & a giant in the industry such as the ones mentioned. these big manufacturers have the resources & budget to do the R&D & materials analysis & testing, where all the smaller china manufacturers can do is what they do best, try & copy a proven design... most times the copy is NOT as good as the original by a long shot... personally I'd want koni or bilstien if at all possible which is why I built my coilovers based on koni touring car inserts...
dual spring rates & progressive springs can be a great thing but will require some R&D & trial & error too, if your not opposed to swapping springs several times in the name of testing then may be better to keep it more simple, but yeah, def a nice place to play if your so inclined...
aluminum vs steel strut mounts??? either would be fine, aluminum will be more common on pillows because of ease of machining, I wouldn't be too critical of either honestly but pillow will most likely be aluminum & rubber will be steel usually...
far as other questions, dust covers are always a good idea to protect the piston which increases service life, camber/caster plates are not usable by our rear suspension design, only front & then probably only going to be enough room for this with pillow which I've already stated my view there... I wouldn't be so concerned about adjustabilty in the mount, camber bolts alone will be more than you'll need up front I'd say & your not going to be changing your alignment on the fly for differing tracks & track conditions, which is what that adjustabilty is really for, again this is another case of street guys copying racers for no real reason... spring preload will be dependent on strut design & fixed cup with locking collar sounds good if you can get it, good luck with it man
