Would be interested it what you thought where the positives and negatives with the mods you carried Miles on your old 4200....
For me the single best mod was the first Mike. A cat failure brought about the decision to go with the Larini Sports Cats. The difference it made to the engine was marked. Breathed more easily and the sound was better without being much louder - better on gear changes up and down. I think this mod had the greatest effect on engine performance - more responsive for the same throttle input and noticeably more 'eager' when pressing on. Frighteningly expensive though, and something I wouldn't have entertained if I wasn't forced into considering it as the "cheap" option.
The braided brake hoses conversely were a relatively simple and in-expensive mod that could be felt, particularly on the track in conjunction with race spec' brake fluid / pads.
Dropping the car on the stock springs was a mixed blessing that in hindsight I wish I'd not done. Looked better and in combination with the spacers did seem to improve the steering characteristics, but on a track or fast road, the car still wallowed and I felt less confident in the feedback I was getting. Wish in hindsight I'd gone with the FD springs.
The FD DBW module had a massive effect, but not always in a good way I felt. Quick but harsh that for an often used car was irksome at times. Great when on a track, but not so much in everyday life. Above 3,000 rpm the GS stock ECU is a match and more adaptable.
The Larini H-pipe and back boxes sounded glorious when on a track or in a gang of fellow Maseratisti, but again I grew to regret their addition when just wanting to get from A to B in a car that is after all a classy GT. I had cabin drone around 3K rpm and they were also expensive.
The combination of all the above made the car very different in it's handling and sound, and looking back on it (and in comparison to my GS) not better, but just different and overall more demanding.
Next time around I'd go for stiffer springs (not necessarily FD) and perhaps the AP Racing brakes along with custom fitted cats, modified but stock back boxes, de-muffled standard H-pipe centre section and after that a custom remap.
Horses for courses and just my impressions looking back on it all...