Possible - yes, if someone would spend high amount of time calculating and field-testing efuse values for 1000mhz, (or even up to 1150 mhz) like freemangordon did for 500-900.

Feasible - no, as 1000 mhz isn't sane frequency to use on N900. Using anything above 900 mhz isn't very wise thing to do RAM limits You anyway, and gain is purely numerological, with unnecessary risk involved.

If You want, You can calculate SR for steps above 900, and test it well - I think that Pali or freemangordon would accept patch for KP, if done properly. But I doubt any of them would interested in risking their devices, to test or do it themselves.

/Estel