At 6'2 and mostly legs, the short toilet my house is a "long" way down from the stp device.. and I'm having a hard enough time with what i currently have to be concerned more with "hitting the toilet" than fine-tuning my aim so that I don't create a messy splash when the stream hits the water in the can...
So I sit down...
I feel like a tool when I do it standing up, because it's a bloody mess that way, but I feel disheartened when I sit down too..
I'll freely admit, the sink has seemed rather tempting for that reason alone.
But! since that's not your problem, but noise is, I'll suggest this.
Put a sheet of tp in the water. It reduces the splashing (not enough for me..

) and the noise, and also, if you're steady enough, learn to aim at the side of the inside of the toilet, so that the stream doesn't hit the water directly, as that'll reduce the noise further.