OK I'll try and explain how I feel the difference the best I can though I may be hitting the nail in my explanation I may not be totally driving it in straight.
The SLE for 1 is too heavy for light wind while you want the sail spread as much as possible to catch as much air which the SLE does, you also want a bellowing effect to capture the air in order to utilize it and direct it better over the sail or it just pulls harder and sheets off quickly and inefficiently.
In higher winds this may not matter with the SLE which is commonly regarded to as a 5 wrap if I am correct, given the chance you get to fly in say 5-10 mph roughly and have the SLE and a 1/4" 3 or 4 wrap fly each for a few and notice the difference.
You can fly a kite in this wind range with an SLE but when you change the LE to say a 3 wrap or even 4 when the sail bellows then you will feel the added control because you are utilizing and directing the wind where you want it for a longer period before sheet off.
What the race rods do is allow the bellow at lower wind speed but also spring back for air capture faster to reload wind, It's hard to explain its much better off being felt I'm sure someone can explain this much better then I can.
With the higher wind the SLE will very noticeably cause an over steering effect the higher the wind gets now granted the rigidity allows the kite to fly in higher winds at the loss of some control now with the 4 wrap 1/4" you can achieve high wind control better or for really high wind you can double up LE's say 3 and 4 wrap creating a so called 7 wrap while still being lighter then a SLE yet still giving that slight curve for efficiency and actually less pull.
Again feeling the difference says 1,000 words, it's pretty much in the aerodynamics of the kite that a lighter more responsive frame will allow you to utilize it better.
For the record people have flown race rods to almost 20 mph anything over 10 and I just switch because its long past the benefits for a particular wind range thats just my opinion.
Now I have also flown a 3 wrap on a vented in 25-30 mph which again 2 x 1/4" LE's would have been better and gotten to the point where the wingtips were almost touching that brought me back to it folding up too much and the kite just wanted to over steer and roll too easily in turns almost to a spiraling effect.
Sometimes I think your better off not knowing all this but then again it helps to have a clue to maximize ones flying comfort and fun.
Hope you can get something out of that because I think I may have overly explained it as I'm reading it going