I got a custom 2.5" exhaust on my 320d. It's straight pipe from the DPF and only one muffler at the end. Sound wise there is a noticeable deeper note, especially after 3000rpm. Doesn't drone at all, power wise it does feel like the turbo spools up quicker due to less restriction, either way I'm sure it's better then having another muffler in the middle.
Paid $500 at a local exhaust shop.
I have a couple videos
If you want it louder next step would be to remove the DPF. I wouldn't waste money on those expensive exhausts, it's a diesel so it will sound pretty much the same. Spend a fraction of the price and get a custom one would be exactly the same.