The Champion book suggests the best spring rate for racing is 300lb/in, and a suspension book I've read supports this figure for a Caterham-sized car.

In my garage I have a big box of springs, ranging from 150 to 400lb/in. Will I get any benefit in moving towards harder springs? Will it increase grip or merely reduce roll? Does it make any difference?

Depends what engine etc. you are using.
300 on the back sounds a bit hard to me. I think if you use 400lb you might need to get your fillings tightened.

David

My Stuart Taylor car has 275 on the front and 150 on the rear - recommended for general road/trackday use. The rears are so low because I have a live-axle.

I guess slightly harder would be expected for a 'racing' car.

My car has 400 on the front and 180 on the rear. I think you can run the front softer if you have a front anti roll bar.

If I were to change the springs I would try them softer on the front and rear. But thats not in the budget just yet!

Gavin