The RC A2 mags are the warmest of the alnicos (soft highs, lots of mids, loose lows). The typical polished A5 is very bright, mid scooped, and has a very big bass.
The RC A5 is slightly warmer than a polished A5 (slightly softer highs and more mids).
Since the Slash already has an A2 mag, putting in a RC A5 will brighten the highs, and tighten the bass, and scoop the mids a bit.
In a 59 (stock polished A5), the RC A5 will soften the highs a bit and slightly add some mids. Putting in an A2 will soften the highs much more, add more mids and soften or loosen the bass. In some guitars the 59 with an A2 gives it a rather flabby low end.
My personal preference for a 59b is an A8 mag, and for the 59n I like an A2 or A4. The stock 59 is a great verstile pup, but it is just too scooped and bright for my taste.