Akrapovic is a titanium system. The advantage of nickel-based alloys (like iconel) is that it has incredible fatigue/tensile strengths at extremely high temperatures. It's density is similar to stainless steel, so there isn't a direct weight savings, but you can use a thinner walled inconel tubing compared to stainless steel to achieve lower weight.

Titanium alloy is the least dense material that is used on exhausts, but it loses more strength at extremely high temperatures than even high temp stainless steels (such as 321). This is why you don't see titanium exhaust manifolds on a turbo car.

I would also not be surprised if the cost of titanium are lower than iconel. Weight between the two materials as used in these exhausts is comparable. With the HJS race cats this system can shoot blue flames and I feel more comfortable with Iconel due to its istance to heat and fatiquing.

As a result the Akrapovic is a slip on after the catalysts. You can replace the catalyst for race catalyst but they will be made out of stainless steel. The headers, if so desired, have to come from another manufacturer.
Additionally, the Akrapovic exhaust is limited by EU noise regulations. The standard Kline has the same limitations but Kline makes a special version for Sam Ahdoot (ByDesign Automotive) that is less restrictive. It still is free of any drone and behaves at cruising speeds, but at full throttle, especially if you have the full setup with equal lenght manifolds and 100 Cell HJS catalysts, it will scream like a GT3. This is because of the tin walls as decribed above. It has a different resonance, i.e. a high pitch (which I love), similar to a GT3.
You don’t get that from the Akrapovic, as the sound of the new GT2 RS shows. It is nice but not exiting. The GT2 RS exhaust is a similar to the setup for the TT/TTS and also manufactured by Akrapovic.
The Kline exhaust most likely adds more torque and HP because it is a complete setup.

Don’t get me wrong. Akrapovic makes one of the best exhaust systems in the world but Kline is very close if not comparable. The complete Kline setup is $13,000. Definitely not for the faint of heart. But with it, and perhaps some ECU massage by Sam, there is no reason to compromise and buy a GT3 RS. You can have your docile everyday car with high WAF, and still beat a GT3 RS on track with an addictive sound.

My $0.02
Marco