The biggest difference in upgrade success I found was on any route that wasn't obviously elite heavy. This seems obvious, but let me explain. Note that this is East-centric.

As a silver, it was still sometimes a crapshoot for flight to FL or any other leisure route, for instance, where as a gold it was a lock. Same with a flight from anywhere to say, LGA at off hours, where as a silver you might get the call, but as a gold it was a slam dunk.

Now, say you are flying during a peak period such as a morning flight CLT-LGA. Then a gold doesn't mean much vs. silver, but as a Platinum you start having some success. Same with transcons.

Perhaps some west flyers can provide some color about that side of the country, such as PHX-LAX.