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Well the record was held by the Flying Cloud, an American McKay-built clipper...but technically NOT an 'extreme clipper'; (less than 40'' dead rise" Her transit via the Cape of Good Horn: 89 days 8 hours breaking her own previous record of 89 days/21 hours! The ship held this record for over 100 years, from 1854 to 1989. The term extreme Clipper was great advertising though in the competitive shipping trade; so the term was liberally applied to all clippers built by Mckay!

Duncan McLean, the marine reporter who wrote at least 161 more or less detailed descriptions of ships launched in Boston and elsewhere in New England for the Boston Daily Atlas between April 1850 and March 1857, defined an extreme clipper as "clippers of 40 inches dead rise at half floor". By his definition extreme clippers ceased to be built after early 1852.
In the midst of the great clipper boom, in early 1851, he wrote "Nearly all the clipper ships which have been built recently, have had 40 inches dead rise at half floor." This was followed in late 1852 by his comment "It is therefore doubtful whether another clipper, having 40 inches dead rise, will be built."
Further understanding of what was happening in clipper ship construction at the time can be obtained from reading the whole section from which the last sentence above was obtained. "Our first large clippers had rounded lines and 40 inches dead rise at half floor; but now nearly all new clippers have hollow lines, and only 30 or 18 inches dead rise. The upright stem, too, has been modified into the inclined, and the length and sharpness of the ends have also been much varied."..."though very sharp and clipperly in the ends, has only 20 inches dead rise at half floor, experience having demonstrated, that great length with sharp ends do not require extreme sharpness of floor. Buoyancy is of more importance, and the speed obtained, especially running free, is more satisfactory. It is therefore doubtful whether another clipper, having 40 inches dead rise, will be built."
This narrower definition has not been accepted by history. The term "extreme clipper" was applied to the clipper ships built by Donald McKay during 1851 even though their dead rises were less than 40". It was also been applied to clipper ships he built from 1852 to 1854, and to similar clipper ships built by other shipbuilders in the same period. It has been applied to clipper ships built with a lengthening of the bow above to water, a drawing out and sharpening of the forward body, and the greatest breadth further aft.

Now days it would probably be called a 'radical' or 'gnarly' Clipper!