Are Boots of Mobility viable on Varus? Since he has no escape mechanism other than his ult would having a greater passive speed prove to be most useful out of all the other boots?
I'd say "not really".
Boots of mobility give you a chance of escaping if you weren't actually fighting.
If you are going to escape from someone, either back off while your team does their job and defends you.
If you mean "escaping" such as "pushing a tower with 5 MIA and run away before they get you", i'd rather go with Boots of Something Else with an Alacrity enchant.