The Ks, the Ps, the rest of the "alphabet soup" of lowering spring options are all fairly similar: they are all shorter, and still BARELY stiffer relative to the stock springs. They are made to lower the car, which addmitedly looks better than the stock OFFROAD look.
That's about it.
The main issue with "ride quality" comes from the fact that people couple these with stock length springs. These cars have a finite amount of suspension travel dictate by the strut and shock lengths. From ride height you have "droop" travel down and "bump" travel up. The shock and strut length is the same (95% of all aftermarket struts/shocks are made to the same OEM length) but the spring now let's the car sit 1.0-1.5" lower at ride height. Guess where all of that length came from? IT ALL COMES FROM BUMP.
One of the main things we look at when lowering cars properly is how much do we shorten the shock or strut housing compared to stock. AST ran into a little problem in 2010 when they made a coilover that was too long - and had too little stroke. We found this out because when we lowered the car the 2" we normally shoot for with coilovers the rear shock would bottom. It RAN OUT OF BUMP TRAVEL. When you do that the spring rate goes INFINTE and the suspension crashes down on the bump stops.
BAM! Ride quality suffers - this is what people on lowering springs usually complain about.
The image above is what we want to see... the S550 OEM rear shock at bottom is shown next to a 2" shorter MCS RR2 remote double. It has much shorter than stock to gain some of that bump travel back when the ride height is lowered. The total shock travel usually goes down a bit, but the relative amounts of bump to rebound remain the same. When we design a shock/strut length we shoot for 3/5ths travel in bump and 2/5ths travel in rebound... because running out of bump is a LOT more painful and detrimental than "topping out" a shock when you run out of rebound.
How do we fix this? Well for the most part - avoid lowering springs with OEM length struts and shocks. We found one S197 Bilstein that was 1.5" shorter than stock, so we made our "Street Pro" suspension kit use these. They've been out of stock for 6 months, maybe 3 more to go... but "that's Bilstein". Otherwise look for a quality coilover kit.
Cheers,
The