I had a 91 740Ti that I put the Bil HD's on with IPD sway bars and kept the stock springs and the ride was not compromised at all compared to the stock shock/sway bars, just less body roll and tire squeel on corners.
When you use the lowering, stiffer springs, the combined dampening rate makes for a harsh ride. Much of the harshess of ride with the faster respone Bil HD's can be improved with good wider and taller tires.
I have the Bil HD's along with the IPD lower springs, 1" sways, upper and lower braces, matched with 205/60-15 tires on my 245 and the ride isn't any harsher than my V70 with sport package 17" 235 tires. Both cars ride very smooth compared to the 740's I've had with stock suspension.
Except for that '86 745 GLE which was pure mush and didn't come with a rear sway bar.
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'89 245 Sportwagon, '04 V70 2.5T Sportwagon