Will a valved exhaust cause back pressure on the turbo?

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Hi everyone!

First time posting on here so sorry for any mistakes. I recently straight piped my MK5 GTI 3" all the way through and although I love the sound it can be a little too loud. Particularly when cold starting it during anti social hours 

So I purchased a Valved Muffler box to install into my exhaust system so I can switch between loud and quiet as I wish. However when it arrived and I had a look at it and I noticed the pipe that diverts the exhaust gas into the muffler when the valve is closed looks rather small compared to the 3" piping. It's also quite a sharp turn. My worry is will this cause too much back pressure on the turbo?I wouldn't drive around a lot with the valve closed I'd only really close it when I'm starting the car with sleeping neighbours nearby etc. I also plan on going stage 2 with the GTI soon as well. Could this be a problem?