I have no trouble using the CS radar with REX.  Also, I see no frame rate reduction when using the Wx radar compared to not using it.  When using a well known payware 737 that has this feature, using he Wx radar came with quite a performance price.  Not this CS 757.  SO I turn it on and leave it on.  Every once in a while I play with he gain and angle settings for the heck of it.  Saw wind shear for the first time today.

Still...realize that no weather radar simulator for FSX is really that good or accurate as it does not appear possible to properly model this function yet in FS. So....mostly eye candy for the most part.