I am currently having the same problem so I hope you get some suggestions that are the silver bullet!
I have tried 4 different types of chin straps and while they help, none seem to solve the problem completely. The best one so far was the cheapest. You can find them on ebay for as little as $2 each. They are just an elastic strap with two holes for your ears, and a Velco fastener at the top.
I have tried the paper medical tape over the mouth, and it does work too if you can keep the tape on. I was a bit reluctant about taping my mouth shut, and you will read warnings about how it could be dangerous if you throw up during the night. But how often does that happen, and is tape any worse than having a full face mask if that occurs.
This is an ongoing project for me, and my attempt tonight is to combine a chin strap and the tape to see if that works.
As for the full face mask solution, I tried the Mirage Quattro and couldn't tolerate it for more than a few hours. It seems to leak and make rude noises that wake me up, and I just couldn't get any sleep. I've also tried the AirFit F20. It was better and I lasted 4 nights with it before I gave up on it. I wouldn't rule out a full face mask solution as many tolerate them. If you are paying for them, try to get one that is returnable if it doesn't work for you.
One minor change that you could try that may help a little is setting your EPR to Ramp Only. That way you get the benefit of the exhale pressure reduction when you are going to sleep, and then it discontinues when you are asleep. With no EPR your treatment pressure may reduce a little - perhaps by 1-2 cm. That could delay the onset of when you start to have a mouth leak, but probably will not eliminate it. To do this you need to set the Ramp Time to Auto. I would set the start pressure higher than where you are now (4 cm), and increase it to 7 or 8 cm. This can be more comfortable when you are trying to go to sleep. Then finally set the EPR to ramp only. This will require access to the Clinical Menu. It is easy to get into. Just hold the bottom Home button down and the round button down at the same time for 3-4 seconds, and you will be into the Clinical Menu. But, like I say, it probably will not be the silver bullet.
Edit: One more trick I intended to mention. Some find it possible to hold their tongue at the top of their mouth, and pushed up against the back of the top teeth. I find it does stop the blow by effect, and you can do it when you are awake. The trick seems to be doing it when you are sleeping, and I can't claim to have that part figured out...