Harshness is not cause by the spring rate. It is cause by how good your damper design to handle bumps. Mine is actually 9kg front and 8kg rear. I've got to experienced 3 coilovers:
1. TEIN Type Flex - 7K F and 6K R
2. Bilstein B14 BSS Kit - 6K F and 5K R
3. Ohlins DFV - 9K F and 8K R
The Tein was ok during summer but harsh during winter. It is slightly firmer than OEM at softest settings (summer). At softest settings however it seems to affect handling. My major complaints are it is noisy and not comfortable during winter.
The Bilstein is also fine during summer but can become unbearable during winter. Since it is not damping adjustable, it is slightly firmer than the Tein. Note that the spring rate is less than the TEIN. NVH seems to be the same feeling but maybe little firmer for Bilstein. The good thing is it is extremely quiet and great in highway driving.
The Ohlins was great even during winter, I still had no experience for full summer season yet since I've just got it last November (before the weather became cold). Up to now, I still haven't set it to the softest settings yet (still have 2 more clicks from softest) but as far comfort is concern and as far as I can remember, it is much more comfortable than my OEM. It was not bouncy and not soft when I make a sharp turn. When I purchased the kit, I was initially thinking why the standard spring rate was so high for this kit, I kept asking myself maybe it will be too stiff for comfort but I was prove wrong. The difference between this and my TEIN in terms of adjustability was night and day. You could feel some difference when you adjust 2-3 clicks whereas for TEIN you don't. Softest to Stiffest was a big difference. So it is not about the spring rate but damper design. My Ohlins seems can handle NVH quite well too.
So I doubt it, even when you change the spring rate, you won't feel any difference, not unless you change to a better damper.