I installed the Whiteline X brace earlier and along with the J's Racing tower bar I bought this morning from a local speedshop.

My assessment:

I did notice a difference with this combo of STB and X brace! Definitely .

The front of the car feels more planted at speed on rather waving highway.
Over bumpy roads at moderate speed the car again feels more planted and less steering wheel jerk (bump steer).

90 degree turns are sharper and more direct; not as vague as before although the S2000's steering is already very sharp. But relatively speaking the steering feedback is definitely more solid.

I highly recommend the STB , X brace combo!!

This morning I had a brief drive with just the STB installed, and the difference with just the STB is minimal, if not nil.

 

IMHO, the X braces stiffen um up nice and the stb is great to lean on but not much else.