The optimum temperature is 86 - 94 degrees and the ideal boiler pressure is 8.0 - 15.0 bar.
This is the range of opinions!
PS. I use 8.5bar and 90 - 94 degrees (total temperature variation top to bottom of the boiler cycle 1.0Bar - 1.4Bar). Note that boiler pressure is not necessarily accurate unless you have calibrated it against a reference pressure gauge.
Bezzera recommend 8 - 9 bar and 86 - 89 degrees.
Most manufacturers of commercial hx manchines recommend about the 90 degree mark.
94C is recommended by people in Seattle but in my opinion must be the absolute highest if not beyond the temperature at which some decomposition of the coffee oils occurs (as evidenced by a black slick on the shot).
I am thinking in fact of dropping my machine back a degree or so.