I have noticed that when making a custom potion, the custom effects don't override the potion's default effect. E.g., when I use the command below, I get a potion that has invisibility as well as blindness, but as I customized it, it should only have blindness.
Here's the command:

/give @p minecraft:splash_potion 1 0 {CustomPotionEffects:[{Id:15,Amplifier:0,Duration:200}],Potion:"minecraft:invisibility"}

In 1.8, custom potion effects did override the default potion effect (which back then was provided with the item id, not the Potion tag).

This potion would only give you speed, but not fire resistance in 1.8:

/give @p minecraft:potion 1 8227 {CustomPotionEffects:[{Id:1,Amplifier:0,Duration:200}]}