Wow, reviving a 5 year old thread! Anyhow, yes, 7/8/9 speed derailleurs all use the exact same cable pull ratio. To go from an Alivio to an SLX will be a nice upgrade. You will have to shorten the cable and housing since the SLX is set up differently (or you'd have some odd cable bend somewhere ...I'd guess). Also, remember 10 speed versions are not compatible.
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