When summoning thrown splash potions, you cannot customize their effects, all they do is splash the ground.

/summon ThrownPotion ~ ~2 ~
{
Potion:
{
Count:1,
id:373,
Damage:16461,
tag:
{
CustomPotionEffects:
[
{
Id:7,Amplifier:20,Duration:3
}
]
}
}
}

This works...