A good example of where the new 'Max Ray Intensity' feature in Vray 3 can come in handy. In the render below you can see the dreaded 'fireflies' or 'speckles' that often plague vray users, especially if using a hdri sky (you might need to click the image bigger to see them).
Simply turning on max ray intensity (MRI from here on) removes these fireflies from the main render (reflection element included above as it is in the reflections that the fireflies occur). You can also see that vray isnt havent to work as hard from the sample rate element and so it is noticeably quicker into the bargain.
The only time I have found that having MRI on can slow down a render, is when doing particularly hard to light interiors where there is very little light bouncing around. In those cases vray seems to perform better with it turned off, but otherwise I would recommend you generally leave it on by default.
Below is the same scene but with embree turned on. A small percentage gain, but again, I mostly leave embree on by default these days.