I just did some experiments:
Playing from MC alone.
Paused MC, playing from web browser.
Playing from web browser unpaused MC.
Stopped MC, played from web browser, played from MC.
I monitored Audio MIDI setup during these trials.
In all cases the same thing happened: MC sets the sample rate it needs and the Mac takes it. The web browser doesn't seem to have any influence on this at all; the sample rate just stays at the last one it was set to. Because MC always seems to set to 32bit integer, the bit depth isn't an issue either.
Something, probably the OS X mixer, is mixing the audio together when both are playing at the same time. This seems to work ok, though you probably wouldn't want to listen to two things at once; at least not very commonly.
So I think it's all pretty much ok and disabling exclusive access doesn't seem to have any adverse affects.
Brian.