how vital is it to match manufacturer's for bars and stems. Is it okay to run a Ritchey Stem with a FSA bar for example or should they be from the same company?
Its fine. Some recommend the use of their combos, like Easton, but its generally fine. I am running a Ritchey comp bar with a PRO LT race stem, but that is changing very soon. Its very very stiff though.
I live in a semirural area on the outskirts of Sydney. The roads here are blue gravel, poorly kept, and I generally do 50-70km rides. I changed from a EA70 stem/bar combo to a carbon Zipp stem/FSA K wing bar, and the difference in vibration dampening and comfort were massive. How much the improvement was due to different components as opposed to different materials is difficult to determine, but the lack of fatigue and parathaesia at the end of ride was noticeable from the outset.