It's fair to point out that Thorpe also swam in a full body suit, which presumably he didn't just do for the style but for the reduction in drag - and he struggled considerably on his attempted comeback when he had to wear just shorts.It tells us how ridiculous Ian Thorpe's time was, when it's pretty much the only pre super suit era PB in any discipline, across both genders, that can't be beaten by anyone today. Paul Biederman beat it by that one hundredth of a second at those crazy Rome championships, but two days later he massacred the WR in the 200m, and set a time which was 2 seconds faster than Thorpe ever swam.
It does seem like 400m was probably a sweet spot where the buoyancy/drag gains from the suits outweighed their disadvantages (constriction of breathing, friction against skin etc).
Martens (Paris gold medalist) has come within three tenths of a second of breaking the record this year, which isn't bad going while in shorts - certainly I doubt the record will last another 15 years.