Well we can compair this to scale racing. Which has harder suspension... the le mans Corvette C6 or the Off-Road Baja truck? Soft suspension will give you more grip because of weight transfer but like I said before a balance of front and rear grip is what your looking for not just straight grip. Harder suspension will give you a much more direct and responcive feel and it will be much more consistant handling which is why its good on smooth tracks. Also a smooth track will more then likley be very hard packed which will give you more grip with the right tires compaired to a dry/loose track. The soft suspension is designed mainly to soak up bumps and keep the tires planted on the track without upsetting the chassis which the hard springs will make you bounce and upset the chassis on a rough track.

I honestly don't know about the spring issue but if you look on NitroHouse they recommend using only the hyper 8 springs for the hyper 8 shocks. Nitrohouse has a good price for those springs too just incase you are planning on buying them.