Frank, there are lots of GA engines are turbo-supercharged. Cessna Turbo 210 and 206, Turbo Saratogas, any pressurized light GA airplane, the list goes on.
Keep in mind that a normally aspirated engine is rated at sea level, as soon as you start climbing, power out put starts to diminish. A turbo charged engine will maintain it's rated horse power as the airplane climbs, until it reaches what's called it's criticle altitude. This is the point where the turbocharger can no longer compress enough air to maintain the maximum manifold pressure. Above this alititude, the engine responds as a normally aspriated engine does, power output drops as altitude increases. So even though an IO-520 and a TSIO-520 are both rated at 310 hp, the TSIO-520 can make that power at much higher altitude.
I think I might have confused you in the turbonomralized vs. turbo-supercharged explanation. A normalized eingine's turbo system will only maintain sea level pressure and no more.
A turbo-superchaged engine's turbo will produce more pressure than sea level. Either way produces the same results. Induction air is forced into the cylinders to maintain sea level power output. Just be aware there is a difference in the two systems.
Typically, a turbo normalized engine is an add on turbo system. The manufacturer has not designed to engine to withstand high boost pressures, so someone adds a trubo, but limits the boost to no more than sea level. The Turbo Alley Bonanza modifications is the hot thing right now. The Cessna Turbo 182's are a good example of an OEM doing the same thing from the factory. The engine is an 0-540, but Cessna added a simple turbo system to it, limits the boost to no more than sea level pressure, and viola, you have a turbo charged airplane.
As far as tuned exhaust... it all comes down to $$$$$$$$$$. Yes, there are advantages to putting a tuned system on a normally aspirated engine (like the 0-320 and 0-360 series Lycomings). However, the pilot operating handbook does not reflect any change in performance due to the installation of a different exhaust system. So, even though your engine is putting out a few more ponnies, as far as the feds and the book are concerned, nothing has changed. One other factor that seems pretty obvious wants you've seen it. The Cessna 172 and Piper Cherokee series airplanes have lots of room available between the bottom of the enigine and the cowling. Perfect for nice long tuned exhaust runners. However, in higher performance airplanes, like a Bonanza or Cessa 210, space is a at a premium under the bonnet. You don't have enough room to for the long runners.
And finally... emissions. No, airplanes do not run catialytic converters. The amount of emissions from general aviatoin gasoline engines is minuscule compared to cars and trucks. Also, since we pretty much have to squeez ever last drop of performance out of our birds, we are running pretty clean as it is. Pilots have direct control of the mixture (at least most of us, FADEC not withstanding). Most operators run 100LL aviation fuel in the US, not sure about the EU. Yes, it still has lead in it. No, I would not consider it a "dirty" fuel when burned.