Tamiya makes two different sets of springs Soft set (3 rates)
and Hard set (4 different rates)
As for as I know (and comparing the kit supplied White w/ Yellow Med spring)
it compares to the Med Yellow Hard set in this kit:
http://www.tamiyausa.com/product/ite...oduct-id=53440
(nevermind the outdated black and white photo)
From that kit, you have red, softer, and blue stiffer, and other white, more stiffer.
I have a set that I bring with me to an indoor carpet track, but, have not used anything other than the kit springs.
I drove an RC car on the High School tennis court once, lots of grip
, but really upsets the P.E. teachers!
As far as gearing in your application, I would probably start with the 28t, (from the 28 and 29t set) A safer approach (as per your manual with the motor) would be to start with the 26t (from the 26 and 27t set)
Check the motor temp after a few minutes, and the length of a race, and adjust according.