Mine has a similar exhaust and no, there is no deep grumbling sound in mine either - especially not over 3000 rpm where it starts being very noisy. At 4500 people start looking obviously worrying what I'm doing to my car. It was measured to 90 dB at 2 meter distance from the rear, 2 meter from the ground, 4500 rpm, unloaded. That's a lot!
I think you can only get deep sound from an exhaust if it is long - obviously the Murena has a very short exhaust which (1) makes it very noisy (2) makes it somewhat high pitched.
A Lotus Elise accelerated out of Lyngbyvej (somewhere in Copenhagen for you blokes overseas) a couple of days ago - from the sound (and acceleration perhaps?) I thought it was a motorcycle, but then I saw it's yellow body. Wow.
I'll look forward to compare sound with your car soon, Henrik!