I'm not really sure where you're getting at with Signori. Stick him through the middle up top for Barcelona and he's netting at least 30 goals a season, probably more. He was a tremendous finisher with a devastating left peg. He never managed to outscore Van Basten, but I'd rank him higher than Higuain for example, who did manage to outscore Ronaldo in 2010.
As for Van Basten's goalscoring record in Italy, after his first injury-wrecked season, it was 47/33, 40/24, 35/11, 38/29 and 22/20. The 25 league goals he scored in 1991-92 was the highest scored in Serie A in almost 30 years. Only that poor season in the middle, but otherwise very tasty for such a defensive and competitive league, in a much more defender-friendly era. Or to put it another way, the season before he retired Milan scored 74 goals in the league. Without his services the season after he retired, they managed just 36 goals. Still won the league but lost more than half of their entire attacking output. And even taking into account the lower standard, his record at Ajax is tasty as well - 26/28, 33/22, 26/37, 27/31. Fundamentally I don't see Van Basten's productivity, or perceived lack of, as a weakness. It's certainly no barrier to him being considered among the very best because people recognise it was a much tougher era to rack up the numbers in. That's why he is regarded in the top bracket of strikers, alongside the likes of Puskas, Romario, Muller, Eusebio, all of whom scored 2-4 times as many goals in their careers.