What you're hearing is coming from more than one source of causes.
I honestly can't say why your exhaust sounds the way it does. It may or may not be fine just how it is. Exhaust harmonics and sound pitches can change in different ways in RPM ranges to no end of possabilities. That diagnostic can't be used very well.
Do you have any other information about what you have changed or done to the car since the problem occured?
Listing your modifications and operations on the car would help a great deal in finding what is wrong with your car, if anything at all.
The backfiring problem should not exist at all. Exhaust scavenging on deceleration is normal to some extent, but this is more than likely directly related to your experiences with loss of power.
The turbo itself can act as a muffler to some extent as it does disrupt the harmonics coming from the exhaust manifold while it changes it and uses the exhaust pressure to run the turbines.
Here are some symptoms of backfiring and poor acceleration and possible causes:
http://www.cygnusx1.net/supra/library/TSRM/em/EM_05.html
I would need to know everything that you have done to the car to help any further. I would also need to know how it idles and runs otherwise besides when the problem occurs.
