I can't comment on the o2 sensor as I have just done this reciently.
But as far as....
I can tell you that there is almost no change in the noise level.
(I expected it might, and was pleasantly surprised when it did not)
I have got 29mpg and 31mpg on my last 2 tanks of gas.
This was done on a 95 SL2 with 170k.
It sounds a little bit deeper, but I can not tell a difference in the noise level.