Will Boots of Swiftness + Tenacity be a good item for Off-Tank champions?

I always love MS.

Though at times, when i'm using a semi-tank type champion, I mostly get frustrated for being slowed/stunned/rooted when i'm about to chase/initiate/kill enemy champs, especially those who love to be isolated.

I want to know if it is worth buying boots of swiftness + zephyr since the boots reduces slow effects and has better MS than mobility which gets lowered when in combat ( not useful when i'm the initiator ) with the tenacity of zephyr/ancient golem if from jungle.

And if I will get the slow reductions from the defensive masteries, will it still trigger with the boots + tenacity bonus?