I have a set of FA500 coming in with all the bells and whistles - swift springs, helper springs, bearings, at 8k/8k. Originally I opted for 8k/9k, due to personal driving preferences, but after reading Iowa999's opinion on why one shouldn't go above 8k for the rears, I changed my specs to 8k/8k.

I'm going to sound a bit patronizing here, so do forgive me. What you should understand, beyond stiffer = more response, is that it's not that simple, especially when it comes to an AWD behemoth like the X. Just the drivetrain alone will cause the car to behave differently with upgrades. Also, stiffer doesn't always mean it handles better. If that's the case, you could very well bolt the car up with no springs or shocks and just ride like chariots of old.

What you say you want (very stiff and handles better) is, therefore, somewhat contradictory to what I believe you actually want (better handling at the expense of street comfort). In that case, perhaps you should look at the FA510s, or if you're so inclined, get rates of 10k~12k.