I love all the other controls for the ARP in the Mod Matrix (especially Arp Pattern) but I like to make a lot of experimental, stop-start kind of stuff and being able to control the Arp Speed by an external device would be excellent. 
Arp Speed Destination in Mod Matrix
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From my experience controls that are not continuous values (they are discrete fractions, much like sync'ed LFO-s), are not considered as candidates for routing destinations, although it can be a candidate for soft knob destination - and you can modulate the soft knob from your VST.
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Thanks for the insight... I'm using a hardware sequencer. not VST, but I haven't messed with controlling soft knobs with my sequencer, thanks for the suggestion.