Personally I have never seen the point in the RBT 6 speed; It is a couple inches longer than the stock 5 speed, so requires some minor mods to fit, which may not be as big a challenge with a race car and no A/C.
The tall 5th I have in my road going ZF is the same tallest gear available in the 6 speed, but if you look at the ratios and shift points and rpm drop the 5 speed ratios are well spread, the 6 speed just closes them up and adds another shift.
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