All of these mods are expensive, as is almost always the case with normaly aspirated engines, and in the end you gain 30-40 hp.
I would say that conversion to catless manifolds is the best mod for gaining power and responsivness. I have done this mod, and in my case it works flawlessly ever since.
All 3.2 V6 had 4 cats, two in the exhaust manifolds, and two in the center of an exhaust system. Also there are 4 lambda (oxygen) sensors. Two in the manifolds, called primary lambdas, and those are important for Air to Fuel ratio (mixture), and therefore essential for engine to work properly. If you buy exhaust manifolds from Wizard or Autodelta, you will see that they already have welded in bosses for lambda sensors. The secondary lambdas are located behind the secondary cats, and their only function is to check if cats are doing there job properly. So once you change to manifolds without cats, those secondary lamdas can sense an error and display it on you dashboard (in some cars it happens in others it don't) - so just in case I installed lambda fakers/extenders infront of secondary lambda sensors, to avoid that. They are very easy to get on ebay and cost a few bucks.
The biggest problem with converting to catless manifolds is the work, because it is hard to access the rear manifolds without taking the engine out, but it is possible. I had it done with engine staying in the car and others have too, but the person who is doing it must be experienced with Alfas v6.
I highly recommend you to take your car to a specialist, who knows what he is doing, not just some random mechanic, even if you will have to drive far away. It is very easy to **** that engine up when you do not know what are you doing.