In addition to what's been said so far, I have two main things to add that SHOULD have been mentioned already. Sorry I'm late to the party, I've been off TB lately due to other things consuming me. I'm here now, though!
Get stiffer front springs and trim the internal bump stops of the front HD struts.
740atl, for those rear spring rates, stiffer front springs are definitely in your best interest.
Your front Bilstein HD's have internal bump stops, so if you are lower than stock, you really should also cut the internal bump stops. This has been discussed somewhere in this forum before a couple times, but it somehow does not come up often enough. With that low of a spring rate up front, if you are lowered much from stock, you are running out of travel and hitting the bump stops. Trimming out two of the three nubs(if I remember correctly) made my car ride so much nicer in daily use over larger bumps and pot-holes(oops!). With the cut lowering springs I've had on my car for more than a decade, the Bilstein HD's and cut spring combo are not stiff enough for how low my car is and they still bottom out at times with a large crash. Ideally I would not have cut the lowering springs if I wanted the best ride and to limit the crashing over large, sharp bumps. Trim those bump stops, get stiffer front springs, raise the front suspension, and/or have the struts revalved.
Also as mentioned, the rear Bilstein HDs are good for a lightweight 242 with cut stock springs and not much more. You already have found that out, though. Revalve them, or keep us updated on these cheap Gabriel Ultras. I've run all sorts of other rear shocks on my car, but somehow I ended up putting the HDs back on. I'm not sure why and I don't know what happened to all my other options(Koni Reds were good).
I'm curious what you'd think if you put the larger rear swaybar back on the rear. That will not affect your bouncing over bumps at all, but it will help the car rotate. I've gone back and forth with stock, medium and large rear swaybars. Currently the car has a 22mm rear bar with the 25mm front, but I want a 25mm again for maximum fun(my rear track width is pushed out more than the front, among other things).
I have the original QSRC kit from Ben and when it was installed I noticed a very sizeable difference in rear traction coming out of corners and a more planted feeling for the car all around with the front not rolling over as much. The ackerman is messed up though and I can't use the quicker steering positions without bad scrub while turning sharper - not good for anything! Now when I get in the car it feels soft as hell compared to the 2014 S60 T5 with the sport package, and even our 2006 Elantra that I put Tiburon KYB GR2 struts on all around. My car needs more damping than those off the shelf HDs can provide. They're MAYBE good for a completely stock car, basically, and not much else. I prefer stiffer. One day I'll go stiffer. I have some stuff in the garage for front coil-overs from Harald, but it's not a priority.
ALSO, if you don't have them, Ben's upper strut mounts with the Luxe-Steer setup are NICE! The stack height may be shorter than your current upper spring mounts, and if they are, that means you can RAISE your front suspension for more strut travel, while maintaining the same ride height. Aka, your car will ride better and bottom out less, even with those measly 250lb front springs!
Blah blah, sounds like you have a nice difference with the Gabriel shocks. I should try some as well for that price, the rear end bounce I get with the HDs drives me nuts.