When I changed gear ratios from the ATX to the MT2 I had to buy the softer spring.

This is a stock part so its not hard to find. One spring is silver and the other is black. Not sure which is which.

Also, locktite on the spring pre-load screw will keep your shift point in tune longer. Also locktite the set screw that keeps the shifter attached to its shaft. Lastly, make sure you tighten the set screw down on the flat portion of the shaft or it wont hold for more than a few shifts.

The bogging is from shifting too early I think. I had this problem before also. 2nd gear would hit too early and the engine would rev too low under full throttle. The combustion chamber would fill with fuel and the engine would bog really bad.