I suspect Red Bull with a Mercedes engine would have won in both the last 2 season, probably not as many as they did anyway, ironically, but it would have stopped these ideas that Mercedes have any control over the customer teams in terms of race on race results. The teams pay good money to Mercedes for the engine. Mercedes aren't doing them a favour by supplying the engine, it's a commercial relationship. The simple truth is the teams Mercedes supply don't have the chassis to compete with the works team. Red Bull probably will. Think back to when Mercedes supplied Mclaren and had their works team. Who was the more successful until the 2014 engine regs change?
That's not to say the playing field is totally level, it clearly isn't. Mercedes are running the latest engine in their cars at Monza but not supplied it to anyone else. Again, the commercial relationship with their customers will cover this. Red Bull will probably want something closer to parity and I wouldn't be surprised if Mercedes agreed. There's only one more season until another engine reg change which will probably benefit the works teams again so why not allow some competition in 2016?