I installed both HO bars and 9C1 rear springs a few months ago, and ended up with too much oversteer. I have been able to adjust tire pressure to accomodate (about 35 front, 40 rear), and end up with just minor oversteer.
The 9C1 rear springs DO give it quite a rake, but that also lets everyone behind you (and that IS everyone) see that big tail hook HO bar. I also had to re-aim my headlights.
I think now I'd prefer softer springs in the back and more equal tire pressures front and rear, but I'm really not complaining.