I have the sport version on my Z4M also. Before I bought my sport version, I sat in my friend's E46 M3 and he had the race version on his M3. The exhausts were much louder, but the drone was also unbearable to me so I ended up getting the sport version. My friend couldn't stand the drone either, and switched to another brand soon after.
I actually had never heard of the sport version in person before I bought mine, and was even worried that the exhaust might be too quiet. The Eisenmann exhausts for the Z4M hadn't even come out yet so no one had them either. I did, however, did a bunch of research on M3 forums and chatted with probably 20 Eisenmann Sport owners and not a single person gave it a negative review.
The sport version definitely isn't loud, but it does sound very deep and much more throaty than stock. You can easily tell it's aftermarket by the sound, but the sound is also not intrusive or ricey. You rev up the engine to 3k~6k and you hear the rumbles come alive. After 6k~7k RPM's, you pretty much just hear the engine noise only. When the exhausts were first installed, there was actually some drone and I almost decided to put the stock exhausts back onto the car after just 2 days. But as the exhausts were slowly breaking in, the drone disappeared after 400~500 miles. It's now been about 2.5k miles since I had the exhausts installed. On the freeway when I'm crusing at 60~65mph in 6th gear, for some weird reason, the exhausts actually sound quieter than stock, which is great when I simply want to cruise relaxingly.