Support with Void Rays and Phoenix (players seem to want to mass tempest only)
Keep one or two with army to force engagement, snipe units, take out tech buildings, get free shots on army outside your base.
Buff/change ideas (each idea is separate, not recommending all together) :
-Long range shots do same damage, but attacks at closer proximity do higher damage.
-Mild aoe at close range
-Energy for a "yamato like" shot that does higher damage (sniping units and building's)
-Available earlier in tech tree, but range upgrade stays toward end of tech tree.
-Debuff caused from attack- slowed movement speed/production, stun (interesting, tempest negates 1 unit from combat...), mana burn, armor decrease splash (similiar to corrupter in BW), etc.
I like the concept of the tempest and think some new strategy or buff's can make it more viable.