I believe the final drive ratio of the S 6spd to be different than the 5spd. Also, consider that in the S, it has more torque (and hp) to take advantage of the additional gearing. Also, 5th and 6th seemed to me to much more like "overdrive" gears than real performance gears, so unless you change the internal gear sets, you might not improve performance at all.
Coming form 911 history, it seemed with each engine upgrade, there were refinements/improvements to half-shalfts/cv joints to handle increased power. Could be the same here.. could be because the 5spd is Audi and the 6spd is "porsche" (getrag maybe) and built to work in 350hp applications (heck, maybe its the same as the TT putting out 450hp!) vs the 200-220 of the base boxster motors. Might want to check if there are differences in the rear hubs/carriers/attachment pionts too.
you mention "racing" - be sure to check the rulebook to see if this mod is allowed.
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1976 914 2.0
2000 Boxster 2.7 (sold)
1978 911 SC (sold)
1970 914 w/2056 (sold)
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