Der går du (og ikke behov for case-testen, faktisk):
SELECT [user-name],
submissions,
[date],
place,
recency,
DATEDIFF(DAY,
(SELECT TOP(1) [date]
FROM [top-design1] td1
WHERE td1.[user-name] = [top-design1].[user-name]
AND place = 1
AND [date] < [top-design1].[date]
ORDER BY [date] DESC), [date]) as recencywin
FROM [top-design1]
Jeg formoder, at du selv kan klare at bruge det til indsættelsesformål.