Foam inserts.

[J.Kirkman]

I currently run schumacher medium foams in all my 1/10th buggy tyres but what effect would running different firmness or brand of inserts make? Are they a tuning option used by racers?
I purchased some medium foam inserts a while back but also had the choice of hard and soft - and thought I'd ask what difference these make and when each type would be preferable.

Any ideas?

Cheers guys.