Thinking your all 3 touching on the potential answer. Jake Brake with variable turbo and wastegate and timing and decel enleanment strategies. I could see a jake type system to help braking. Variable turbos and wastegates that help keep it spooled up. The different sound may be from all 3 types of strategies. I could see the decel fueling actually being on a richer side. So not to lose your port wall fuel film. When they get back into throttle because a higher boost. The occasional backfire was probably a slight lean condition. At that instant of rpm kpa timing and boost. Because of lack off fuel film from off throttle. Like Andy would love to see some of the technology and strategies they use for those cars