Better Call Saul" is not known for using fancy-pancy effects just for
fun, it's all realistic there so I wondered what was the reason for
using this sound effect? It gives a huge touch of evil to Gus and the
scene but we already know who and what he is, so in this regard there
is no new information for us.
As you've stated the effect allows for the audience to see/feel Gus in a more evil light. It's true this is not new for Breaking Bad fans, but the point of the scene, or really the season, is not solely about new information, but rather finding a way to "bridge" the two stories together, as season is beginning to "overlap" into the Breaking Bad reality according to the Eps.
“This is the best season yet,” Vince Gilligan told Comic-Con today of
Better Call Saul’s fourth season that starts next month. “The overlap
between Better Call Saul and Breaking Bad gets closer and closer,”
added the creator of the Bryan Cranston-Aaron Paul Emmy winner and
co-creator of its prequel of sorts, driving the cavernous Hall H crazy
“You are going to see some stuff,” Gould cryptically declared before
showing a clip of the Bob Odenkirk-portrayed slimy lawyer who first
appeared on Breaking Bad in its second season in 2009. “You want the
last hill on the rollercoaster to be the scariest of all,” Gilligan
If one has noticed, the first episode of season 4 didn't feel like Better Call Saul at all. Jimmy was nearly non-existent due to having a complacent demeanor with little dialogue; Nacho's scenes were super intense; a Breaking Bad character turns up for a cameo or two, and the score was closer to a Breaking Bad score. Really only Howard's never ending "speech" and Mike's warehouse stint were the only shred of comic relief that Better Call Saul leans more heavily on. It was also just stated in a podcast that the future scenes are actually right at the tail end of Breaking Bad between episodes 5.15 and 5.16.
In addition episode 4.02 also had another scene with Jimmy were he was a bit more over the top and self sabotaging than usual, closer to what will be Saul Goodman. We also saw another character blow up in a way we never have before.
It makes sense that the writers would have to blend tones of these shows together to better bridge them for the sake of both stories.