It depends on what revision GB board you have. I have a ga-7vaxp rev.1.1 and had to do the L3 mod on my 2100+ to get the lower multipliers to work. A friend of mine has the rev. 1.2 and the lower multipliers work fine on his. (they changed the fsb switches on that rev.)
Update 6/30/02:-
Deerhunter's Thread at xtremesystems.org reported closing only the rightmost 8X Bit Value L3 bridge with a conductive trace around its cut, no insulating fill, then booting to 5.5X with Multiplier control from 5X thru 12.5X available from bios, but 13X and higher not working.