The trouble with talking about exhaust is that everybody knows that "back pressure" is bad. Modern exhaust engineering passed this stage a long time ago. To understand exhaust flow dynamics you must forget "back pressure".
Start with the engine. The intake valve opens. The combustion chamber fills.The mixture ignites. Then the exhaust valve opens to let out the burned mixture. This is how it worked in the Ford Model T. In these circumstances you controlled exhaust flow with the length and diameter of the exhaust pipe. The aim was to let out (scavenge) the right amount of exhaust so as to empty all of it from the chamber. This makes room for the incoming charge.
In a more modern engine there is valve overlap. To some degree the intake and exhaust valve timing overlap. Now exhaust flow management becomes more complex. You have to scavenge the chamber, but intake is occurring during exhaust so some of the intake charge dumps out the still open exhaust valve. Now you need an exhaust that will accelerate scavenging so it is over at the time the intake valve starts to open. Then the exhaust system must utilize the exhaust gases to form a vapor barrier, at the still open exhaust valve, to stop the intake charge from flowing out the still open exhaust valve. You can do this with vapor or resonance waves, or a restriction that will stack or slow the exhaust gases for the period the exhaust valve is still open. The next step is to adjust all these factors to achieve maximum efficiency at the maximum flow capacity at the maximum RPM of the engine, or where ever else the engine will be most used.
Modern exhaust systems are designed to form a low pressure area at the exhaust valve when it opens. This low pressure accelerates flow from the chamber. The reverse wave vapor barrier is formed at the still open valve to optimize filling the chamber. One problem is that what works at 10,000 RPM doesn't work at 5.000 RPM, and vice-versa. Also, the closer the exhaust system gets to maximum power potential, the narrower the RPM band it will work in.
Stock exhaust systems are tuned to give a wide range of usable power. Can it be improved? Certainly, but it almost always involves a compromise that robs power and/or ride-ability from one RPM band to get more in another RPM band. Adding complexity (and expense!) in the form of a variable valve in the exhaust system can overcome a lot of this. Combined with fuel injection/management systems you can get a tractable motor with more power than with a non-variable valve equipped exhaust system.
In a 5,000 RPM motor this is overkill. The engineering costs can't be justified for the little bit of extra power an otherwise stock RE engine can produce. The stock RE system works fine for the performance parameters of the engine.
Things like the flapper valve that was suggested have a certain amount of theoretical attraction, but in practice are problematical. I had a similar device on a muffler I used on my dirt bike in the '80's. It simply disrupted flow. The engine felt torque-y, but it was God-awful slow until I was waaaay up in the RPM band. Acceleration sucked. I suppose a variable spring/ramped actuator might have helped, but then it would have been more complex. I changed gearing to accommodate my torque application needs and went back to my older, more efficient exhaust.
Remember, it is not about "back pressure". Modern exhaust utilize flow dynamics and wave theory to give good power over a wide range of use.