Since my current thrasher fit is pretty much public record now I thought a little discussion regarding this ship and it's fits is appropriate.
The first thing to do when approaching a Thrasher is to understand the design philosophy behind the class and the Matari variant of the same. To do this let's look at the bonuses and the slot layout of the Thrasher.
Engineered as a supplement to its big brother the Cyclone, the Thrasher's tremendous turret capabilities and advanced tracking computers allow it to protect its larger counterpart from smaller, faster menaces.What does this tell us? The first thing is that it was intended to kill frigates. It's bonuses back this up. Extra tracking and small projectile turret damage are good against frigates. Then there are the class bonuses penalties. -25% rate of fire and 50% bonus to optimal. The first one means this ship is intended to have the damage output of a similar ship with 6 high slots, The difference is that it's supposed to do it with a bigger alpha strike (more guns, firing less often). The second is clearly aimed at long range guns. As a rule of thumb, optimal bonuses benefit long ranged weapons and falloff benefit shortranged. With a whopping 50% bonus to optimal this ship is designed for long range (from a frigate's perspective) fights against frigates.
Destroyer Skill Bonus: 10% bonus to Small Projectile Turret tracking speed and 5% bonus to Small Projectile Turret damage per level
Penalty: -25% Rate Of Fire for all turrets
Bonus: 50% bonus to optimal range for small projectile turrets
The slot layout is 8 highs, 3 mids and 2 lows. 7 turrets, 1 launcher.
Now the thing to realize is that fitting a ship counter to it's bonuses is do-able but suffers in contrast to fitting a ship in accordance with it's bonuses. So let's thing about this.
The first thing is that this ship is designed for long frigate range combat. This is ranges from 20km to 30km on up as you hit T2 weapons. So forget about fitting tackle. Yes I know a solo destroyer that wants to PvP should still fit tackle, but the general lack of maneuverability and tank means that soloing a destroyer is at a disadvantage right off the bat. This ship is not meant to be a solo boat. It can be done, but there are better options. It does however need to control range against smaller faster vessels (as much as it can anyways) so at least one of those mids should be an MWD (smaller frigate weapons can't do more dammage to you with it on than with it off so might as well have it boogying you arround) I'll get into when to break this rule. This leaves 2 mids and 2 lows for your "specialization" modules.
The 7 turrent highs are easy: most of the time just fit a full rack of 250mm's. The 280's are used in specialist gank fits. Most of the time the extra tracking on the 250's make them a better choice.
The 8th high slot has a lot of options: Missile launcher, remote repper, neut/nos, salvager, smartbomb, rocket launcher with defensive missiles, empty. Choose something that will fit after everything else has been decided.
The 3 mids: Unless you're a specialist gank ship, an MWD in the first mid is almost mandatory. Unlike a tackler you're not trying to close, you're trying to keep the distance open. After that you've got sensor boosters, shield extenders, remote dampners, target painters and targeting computers as the standard modules of choice. Here it's very much up to personal choice and your vision of exactly what you're going to do. They all have applications and they are all useful. A squadron of dessies can mix their fits to provide various options.
the 2 lows: Gank or tracking are prety much your options here. Depends on your prey. But you will just about never go wrong if you fit 2 Gyrostabilizers if you can't think of anything specific.
Look, These ships are all about the ranged frigate kill. Fit and fly accordingly as part of a gang. Realize you'll die quite quickly if engaged up close - don't get close. There's realy no way to fit an effective tank on a destroyer. So don't bother - go for the gank as much as posible, use a range tank. A full on T2 gank Thrasher with a pilot that has destroyer 5 and advanced weapons upgrade 5 can put out a scarry amount of alpha at 60-70km (this is the one exception to the always fit an mwd if you can rule - it needs the 2nd sensor booster to range out that far). It can two volley most interceptors. A full on tracking fit has almost as much tracking as an autocannon frigate. This means you can actualy reliably hit ships that are orbiting you at tackle ranges (it's sweet spot is arround 10-20km right where long range tacklers like to tackle) and is even usefull at close range if you turn with your oponent to reduce traversal.
The are some subtle changes you can make to the ship to tweek it one way or the other. Trading range and gank for tracking and ability to hit reliably. But prety much under no circumstances should you try to tank this beyond a single extender in the mids. Dont' fly this ship up close, don't tackle with it. In wolfpacks it's all about the massed alpha. In fleets it's all about the anti-tackle and anti-interceptor. Against frigates, if you start out of range you should not loose 95% of the time. Never approach a target head on, always at an angle so as to be prepared to break off to keep the range open. Other ships are tackle. You're the anti-frigate alpha.
When setting up a warp in point defence (in FW) put tackle on the warp in point and setup with the warp in point at your optimal. For gate defense either setup on the gate (if you're closer ranged and tracking fit) or at about 40km-50km (if you're T2 range/gank fit). Again you're not tackle - that's someone else's job.