I haven't done any testing along these lines, so I don't know what the actual answer is, but I don't think adding more Arc instances than you have CPU's would do anything except use up more memory...
edit: though I suppose it's possible that garbage collection times might not scale linearly... if it takes more than twice as long for the garbage collector to run on twice as much data, then a larger number of smaller processes might give you some kind of performance benefit.
