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Is there a straight-forward way in SQL syntax to write this in a single query without a sub-query? And is there any chance in a performance-gain that way?
SELECT ProductCount, COUNT(*) as Occurence
FROM (
SELECT OrderID, COUNT(DISTINCT ProductID) as ProductCount
FROM OrderLine
GROUP BY OrderID
) X
GROUP BY ProductCount
ORDER BY ProductCount
What you have in your question is the recommended way to do it.
You are unlikely to see any performance gain from any change. This is a very common pattern which your database should cope with just fine. If it is slow, consider adding appropriate indexes.