v. s. rearsets shifter

[Dvsrd]

What effect does extending the linkage have on throw distance?

None. If you mean the linkage rod.
(Think of it as a chain drive. Chain length does not influence the ratio. Sprockets do)

[Czakky]

 

[soonerbillz]

One of the best reasons for GP shift for racing is it virtually eliminates missed shifts.
The downward toe action is much easier and stronger to make vs the upward toe strike practically garuanteeing positive engagement.. All my race bike were converted to Gp and I rarely if ever had a missed shift.

I have read that the main reason was to be able to shift up while still cranked over, like accellerating out of a lh turn. With regular street setup, the boot toe would drag.

You are correct.. that is why it's used in racing primarily.
My statement was strictly a opinion (that others have agreed too) but only a secondary reason..not primary.