I haven't confirmed in depth that the front / rear height ratio has changed, since i haven't noted the height before the modification.
Just keep in mind that lowering the rear ride height also lowers the rear GRC, and this can impact handling and steering response. When lowering I think it's generally advisable to ensure the rear lowering is no greater than the front lowering, or less than the front lowering. I would always keep lowering ambitions modest, as lowering usually messes with the geometry in ways that may not be obvious.
Plus, there's another variable on this issue that i didnt mentioned before. I've powerflexed the rear end some days after the suspension upgrade. It was part of a maintenance solution and i'm pretty happy with the feel plus already passed two years and fortunately the rear camber is still the same today as it was the day i've made the bushings maintenance. The powerflex bushings on the rear turned it more precise, stiff and grippy.
I've fitted custom made rear lateral control arms on my 147. These are based on Toyota Corolla and Camry rear control arms, and eliminate the relatively soft Alfa control arm bushes (note that my cars' 'original' rear control arms appeared to be in good condition, but may have been aftermarket and not OE, and there is some suggestion that at least some aftermarket rear arms have problematically softer bushes than OE arms). Each control arm is a hybrid of half a Camry arm and half a Corolla arm (one or both shortened to suit the length required for the Alfa). The Camry arms use a spherical bearing at the outer end (factory part, fully weather sealed etc). The Corolla arms have rubber bushings, but much stiffer than the Alfa bushings. So, each hybrid arm has a spherical outer bearing and a fairly stiff rubber inner bush. The arms are also tubular, eliminating the flimsy 'U' shaped Alfa arm stamping. A search on this forum for 'Camry' and / or 'Corolla' should find a more in depth description. These arms were a huge improvement in rear end stability, probably similar to what you have achieved with fitting poly bushes (maybe better, can't really say).
It could be this that is making it less grippy at the front in comparison. One thing is fact, i feel more understeer in initial turn and less after the initial turn. Plus with the 2.4 bar F / R not only the front lacks response as the tyres get chewed pretty easy. My last Toyos got totally bald on the interior in 8000/9000kms while the center still almost new. Now with 2.6 front and 2.4 rear the new Conti are wearing evenly and the initial turn in understeer almost gone.
Personally I've found that very high tyre pressures work very well to sharpen up the handling and steering response (what most people would probably regard as 'excessive' pressures...). The pressures I'm using are substantially higher than you are using (arrived at with a lot of experimentation). The down side is harshness, but the car is much more fun to drive.
My cars' steering response is good with some toe-in, and the tyre wear is very even across the treads. Toe-out will promote inner edge wear (when negative camber is also present). I'm using 3mm of toe-in both front and rear, and prefer the steering feel of toe-in compared to toe-out (which I find makes the car a bit 'wandery' without really improving turn in response, despite what seems to always be said regarding toe-out...).
Supposedly this B12 should work fine and the rear / front measures are according to what is expected after installation. So thinking once again about it, it could be the rear powerflex stiffening everything up influencing on the geometry response between back and rear. I have planned to powerflex the front as well considering that i'm so pleased on the effect and compliance on the rear. I might have another reason to do it now.
Softer bushings in the rear control arms adversely affect steering response, as well as handling 'precision'. Also, when the bushes are laterally loaded the bushes in the frontward control arms (front arms of the rear suspension) compress more than the bushes in the rearward control arms. This is because the arm geometry
laterally loads the frontward arms more heavily than the rearward arms, not because the bushes in the front arms are softer, all eight bushes in the four arms are same stiffness (many cars also use softer bushes in the front arms compared to the rear arms). This creates a passive steer effect which induces rear toe-in when cornering. This passive steer is understeer inducing (i.e. 'stabilises' the handling by inducing understeer...). The softer the bushes the greater this affect will be.
I have standard bushes in the front suspension lower control arms. I'd like to try stiffer bushes (poly being the only practical option for this), but it's a lot of expense for something which may cause more trouble than it's worth...?