After much messing around with ToString() and getutcdate() it seems the DateTime field is stored in different formats between the various statistic tables, though via SWQL Studio all dates are presented in a consistent format.
Modifying the query to format the date YYYY-MM-DD HH:MM:SS outputs the filtered results correctly.
SELECT CPS.DateTime, CPS.Status, CPS.CustomPollerAssignmentID
FROM Orion.NPM.CustomPollerStatistics CPS
WHERE CPS.CustomPollerAssignmentID = '9b7d2e83-671b-40f1-9189-09f51c66670e'
AND CPS.DateTime > '2015-03-17 21:32:00'
* it would appear this also occurs in Orion.VIM.VMStatistics