me and dan both have the same braces, front and rear,
yes they do make a diffrence, i have the oem front ARB and NO rear arb and oem springs and shocks (at the moment), the back end is in check all the time (more than it used to be any way) and feels so much better on the twists, the front end is planted too.
i did have the engine cover on mine and it was fine but no longer have it on due to some issues i had with coil packs
i can see how much the rear has made a diffrence since installing it, my old cupra used to be like an MX5 in the wet on bends, if you check my RR you will see how the LC ended up on a more or less straight wet road when the owner moved out the way of an animal...
i got so used to the "scattyness" of the leon i actualy liked it and got used to a little bit of "drift" like action to fight, the LCR was a lot better but with the braces fitted, i have no fuss at all
the guys with ARB's and the rear braces must be loving it more than me as they must be solid!!!