I had XWX's on my queen mother when I first got it; I thought they were terrible, but they were also ancient. The problem, at least in our region, was that Michelin did not consistently produce the XWX in the sizes used on vintage Ferrari. You would hear stories of folks buying "new" XWXs and then discovering that the date codes put the tire at 4 years old. I went with Pirelli P4000s and was very happy with them.
There is a gentleman on tomyang.net who has owned his 330GTC for over 30 years and has put over 110,000 miles on it. this is what he had to say a few years ago about the tire question.
"I have been driving a 1967 330 GTC since 1975 and, having put over 110,000 miles on it, I can claim reasonable experience.
I’ve used the following tires: Pirelli (CN-72 as I recall), Michelin XWX, Goodyear Eagle GT, B.F. Goodrich Comp T/A, Yokahama Y(???)VR, and finally Pirelli P-4000. Since I had my own tire changing equipment, I could experiment as I wished.
When running tech and instructing at FCA track events in the 70’s and 80’s, I spent a considerable amount of time on the track, pushing the GTC to its limits. The absolute best all around tire I used, both for track and road was the Comp T/A – 205 or 215/70 VR 14 – I can’t recall the size, but I do recall that although it worked well, it did not last long. (I also used the Comp T/A on my 308 GT/4 and they were fantastic). The CN-72 was ok but the XWX was the worst tire I ever put on the car. The best road tire, as well as the longest lasting was the Yokahama.
Although I no longer participate in track events, I still use the GTC as an everyday car and drive it aggressively. It has around 160,000 miles on it now and it’s working quite well with the Pirelli P-4000. They are 205/70HR-14 and I have never bottomed the car. Since it’s equipped with Borannis, I must run inner tubes. I keep around 35 PSI in the front and 36 to 38 in the rear and I have yet to have them let go when cornering hard. Wear seems pretty reasonably but time will tell."