as a data point, ive been running a 266 (66 FSB) @400 (4x100) for years now, stock retail HSF w/NP. YMMV, but yeah, most of these seem to oc quite high, well over 400...like Nack said, these are probably "remarked" 400's to begin with (400's or higher that tested on the outside of the statistical stability curve, and remarked/certified as 266's).
[Edit] Oh yeah, forgot to add, it is one of the .25 die cores.