Well, if it has to be done (which I'm sort of on the fence on), I think there are a few ways to do it. Since I don't tend to play with Wolfpack Tactics, your "standard" Tempest doesn't seem to be massive outlier but I can agree that if you really gear for it, a Tempest is terrifying.
1.) Replace HEF (as has been mentioned). The Active Flare Launcher makes some sense but it feels...lame. What about Reserve Deployment? Toss another Drone out to temporarily increase its anti-Fighter/Missile power with a tiny bump in overall firepower against a harder target. I really don't want to lose the Terminator Drones because they are one of the most unique additions to a ship in the game.
2.) Reduce overall firepower. Make one of the Medium Energy Mounts a Small. Part of the issue is that SO Tempest can double Heavy Blaster for some truly terrifying firepower (especially with HEF). If you take the 2nd Medium away, it simply can't match the firepower of a Destroyer.
3.) Tweak the flux stats down, or reduce its overall speed. I guess this is the most straight-forward.
4.) Increase DP/maintenance costs to 10. If it's going to have the firepower of a Destroyer, make it cost like one, too. It's still no Hyperion but if my choices are "Tempest or Hammerhead," that's a bit of a decision to make. In general, I think Tempests need to be much rarer to find, too (no matter what changes are made).