Thinking about adding a BOV on the GLC63, any thoughts or experience with it?

There are a lot of threads on here discussing them.

They are going to sound good as long as you don't mind a "ricer" sound.

Venting to atmospheric pressure will likely cause a slight turbo lack as compared to stock.

They are cheap so try it and see.

All that being said, you could just unclamp both hoses at the bottom and plug them at the intake end and see if you like it before spending money on a cleaner installation.