I am confused by the word ride quality. To me that means a nice smooth ride yet you say they are stiffer. Do you mean it corners better?
XREAS is designed to corner flatter (less body lean) around curves/turns because the shocks are hydraulically cross linked. That's the whole idea of XREAS in the first place--it allows the vehicle to corner flatter without having a stiff/firm ride. And that's what I love about it.
The 5100s & FJ springs make the ride more harsh (firmer) than the stock XREAS setup. I don't like a firm ride on my SUV...I'd rather keep it soft and comfortable. I'll keep the firmer ride for my car, which is designed for performance, not comfort.
That's why I'll switch over to the Limited shocks when my XREAS shocks need to be replaced. I'll lose the flatter cornering, but I'll retain the nice stock ride quality.