They make a big difference. Even for my son who has only been racing for a small while can tell the difference.
I use a mazda speed 6. Mazda 6 (not speed 6) gives a neutral feel for the car if setup correctly. Ltcr has much more aggressive steering but less on the rear. Mount a shell more forward by 5mm + will also give more front and less rear. Try to be consistent when cutting your shells so you can understand the different handling characteristics.
Below link is a good guide.
http://demonpowerproducts.co.uk/faq.php#32
Bed time.