What unusual effects work with the slider

As nimcap answered, unusual hats are just hats with special particle effects. They have no stat benefits, only visual effects.

The only way to obtain them is to trade for them or to find one in a Mann Co Supply Crate. Crates have a low chance to contain an unusual hat.

According to data gathered by users on the forums, it seems that series 1 crates have about 2.45% chance of containing an unusual item, and series 2 crates have about a 1.5% chance. You can tell which series of crate you have by reading the tooltip.

To open the crate you need to buy a Mann Co. Supply Crate Key, which costs $2.49 in the store. If you'd rather not spend money you can trade the crate to someone. Items that come out of crates are craftable and tradeable.

You can find more details about the drop rate on the TF2 wiki page about crates. Keep in mind that this data is gathered by players and might not be totally accurate, but it's probably in the ballpark.

Taken straight from the wiki:

Multiplayer Decal Limit

The maximum number of decals that can exist at once. Decals are used for visual effects like bullet impacts. Higher numbers will result in more persistent decals, at the expense of performance.

There is a command that represents the Multiplayer Decal Limit, noted by the command mp_decals x with x denoting the amount of decals you want visible in game.

Since decals are responsible for showing sprays, bullet holes, and other particle effects in game, reducing x will in turn lower the amount of particle effects. X can be a number from 0 - 200.00.

If you'd like to have some particle effects (to still show bullet holes, useful for learning recoil patterns and such), an advisable amount is mp_decals 9, which is enough for seeing the Scattergun's spread without sacrificing any performance. In Chris' words (creator of Chris' configs)

maxframes.cfg

mp_decals 1 // `9' is a good value to still see the spread pattern from a
            // scattergun without any real performance loss.