Every wax I have tried looks pretty much the same to me on Silver - but to be honest I have not tried anything more expensive than my Zymol Titanium on it, not tried the Concours, Destiny or any of my 3 Swissvaxs on it as when my I spent some time PCing the car about 18 months ago and even that made little difference to how it looked just after wash let alone playing with waxes. Any wax that gives a deep gloss on Darker colours should give the same level as gloss on Silver, but due to silver reflecting light rather than absorbing it you wont see it - I strugle to justify using costly products that you cant see, hence P21s ocassionaly in the summer, Megs 16 winter, and Optimum Car Wax after wash.
The Zymol Titanium did not look any different to to the P21s when I tried it, though lasted longer. Would be interested to see Onyx on it as for the price IMO it is the best value wax out there at the moment and has looked great on all the other colours I have tried it one.