I had all the same exhaust upgrades on my old x-51 car. The headers will yield a noticable difference in torque and sound. In my opinion, the exhaust didn't sound that great with just the xpipe and PSE mufflers. I added the fabspeed long tube headers and was very impressed.

 

I have fabspeed headers installed on my car. It goes fabspeed headers out to fabspeed xpipe catbypass to fabspeed sports mufflers. To be honest i love the way it sounds. Ive done a muffler delete and it was just nasty and annoying in my opinion. Honestly make sure you change your intake like i did. I use to have the factory air box with bmc filter and i like my competition intake from fabspeed. Goes great if you have x-51 or not. Ive upgraded my throttle body and plenum to an 82mm so it pulls really nice.

 

Ok, then light weight wheels would be my suggestion, along with a proper alignment (unless you already do).

 

When it comes to an all motor build, you're going to be shelling money out for little gains here and there. Your best bet is to invest in the other aspects that will help change the entire dynamic of how the power you're making is used. Look into your chassis itself for upgrades to make it more rigid.

Since its a weekend car, maybe look into an integrated roll cage to reinforce the suspension points to allow the KW3s to really show their potential. Look into aftermarket seats to cut weight, and help hold you nice and snug to transmit more feedback. If money is no issue, take a look at Technocraft [100% carbon fiber bucket shell, weights 2-5 lbs without the cushion]. If ~$2,000 a seat isn't appealing, look into Sparco. From there, as was mentioned earlier in the thread, lightweight wheels will be a drastic change in regards to acceleration, deceleration, and turn in response. At that point, with some weight removed from the car, and new wheels, get a proper corner balance alignment to take the maximum advantage of your KWs.

In terms of driving response, you're not pushing insane amounts of power to need different axles, however an upgraded LSD might help with cornering power input, and a gear swap can play into advantage in regards to acceleration [with top speed loss]. Final changes would be a clutch swap for a lighter flywheel to increase response, as increasing the stage will only add the cons of a more difficult engagement to hold the same power.

Pulleys will help increase response, however I always recommend doing a flywheel at the same time as it can create an imbalance that robs you of the full benefits that come along with the upgrade.

 

These type of questions come up often, and the reality is that the car was designed to be a certain way and there are certain things you can change, to make better, (that is a matter of opinion) and faster. You say you feel something is missing, maybe you need to go visit a few dealers and test drive a few different cars (newer Porsches and other brands as well). Maybe you can figure out what you think your car is lacking that way. I own a lot of cars (I have bought 9 new ones just this year), and they all serve a certain purpose for me. Every car I own is modified (even my dogs 2016 Jeep Grand Cherokee is ), and just about all of the 449 cars I have owned in the last 30 years has been. Sounds to me like you do not want to make changes to the car that might adversely effect its value. I think the LSD is a good move, seats well that is a personal thing, I do not know your physical size (I am 5'8" a bit over 150lbs have no problem fitting in racing seats). I always tell people this, before you start to modify your car, learn to drive it, meaning turn off the nannies(is your car AWD or RWD?) and let the rear drift out see how good you are at pulling it back in (this should be done at a large empty parking lot if you are not at a track). You see in my 45 plus years of experience in modding cars I find most people just start changing things, just for the sake of changing them. I am not implying anyone here was done that but some changes are not always a good idea. Good luck!

 

Something as simple as some RSS semi solid engine mounts can make the whole car feel more alive and responsive. You will get a slight increase in NVH though. However I agree with MrPrice you need to workout what you think is missing, otherwise you could end up wasting $1000's till you know what it is.