It seems like using light blue Kyosho spring front and rear is THE setup for any 1/8 scale these days. Kyosho and Associated uses them. I've seen Mugen and Xray use them. I own a few K cars and have all the springs, so I figure I would do some testing.

I did my testing at Revelation Raceway, a large 1/8 track with 200 ft straight, huge jumps, fast but technical, hardpacked with a loose loamy/sandy top layer.

First off, by feel alone, the light blue springs are significantly stiffer than stock springs. I did not like the feel of them front AND rear. Suspension was too stiff. It didn't handle the rough bumpy surface that well, and the rear end was a little unpredictable. I did like the blue spring up front. A stiffer spring up front felt great, but I ended up going back to stock rear spring. The stock rear spring drove the same as a Kyosho orange spring. The two feel almost identical.

So in the end, my fastest lap times came from changing the stock front spring to something stiffer (in this case, a Kyosho light blue front spring), and keeping the rear end stock (or can use Kyosho orange springs).