Putting it in the bigger tank won't really make it skim any "more," as you're implying. You shouldn't have to empty the cup any more than you already do. The skimmer is rated for a 30g tank b/c that's what the experts have rated that this particular pump in the skimmer can handle. Put it to ya this way: Suppose the pump in there is a 120gph pump. The manufacturers have decided that this pump can turn over a 30g tank 4 times an hour, so they're happy calling it acceptable for up to 30g tanks. Take this same 120g pump and put it in a 50g tank, and you have less than three times tank turnover per hour. It will take this skimmer longer to filter out the same amount of impurities, and those impurities might break down into ammonia while they're floating around in the tank for the longer period of time.
Does that make any sense? The short answer you're looking for is really, "Yes, you can put it in a 50g tank," but don't expect it to be as efficient as a larger skimmer. If you're using it as an interim tool until you can afford a larger skimmer, then I'd definitely say go for it.
On a personal note, may I ask how this skimmer has been working for you? I'm going to be getting one myself soon for my seahorse tank. It's the perfect size, and although my 37g tank is a little over the recommended tank size, the small tank footprint limits my options. I'd love to know how you felt about this skimmer!