From what I have heard wall-to-wall is basically how tight an alley you could fit in and still make a u-turn (so it takes into account the overhangs and bumpers that would hit the walls).
Curb-to-curb is how tight a circle the wheels/tires can make. So you could make a u-turn that small as long as the barriers and curbs aren't tall enough to hit the body of the car.
There is turning radius, turning circle, wall-to-wall, curb-to-curb, etc. I think that alot of publications are sloppy about publishing those numbers because I have seen many cases where the number printed doesn't match the description. I wish there was one standard for which number to publish, but instead there are confusing (and sometimes misleading) numbers being thrown out there.
The major difference of the L-sport is the final drive ratio.. Everything else you can pretty much get it at any local performance shop that somewhat caters to Lexus owners. Performance parts are pretty abundant in Japan for the Alzetta/Is300 so I dont' think we have to wait for the L-sport from Lexus if you want to upgrade your exhaust/header, chip and suspension.