The reinforced sidewall will help your turn in, especially in the Focus which is not particularly light. Under hard lateral load the sidewall can roll over, which screws up the contact patch and therefor your grip situation, a stiffer sidewall helps to mitigate that. They will also ride stiffer on the road, if they're only track tires there's no big deal.
2004 SVT Focus
Custom front-bumper-mount bug guts; precision sprayed brake dust on OEM rims; 15% Pine Tree pollen over the clear coat.
No more SVT autocross, I drive the thing in the avatar.