An interesting question :)
Technical monosyllabic answers of either YES or NO, would have to be complicated by all of the following:
Many manufacturers have, and some still do, set the pump bypass pressure to somewhere around 12 bars.
I dont know why, but they do.
Do the machines work?
Can the machines be improved by lowering the pump pressure to within the figues specified in the specification for what constitutes a proper espresso?*
Depends on whos wants to do this, what he has been reading, how much he believes, and the* miriad of variables involved in making a good coffee.
ie someone that doesnt have a good handle on making espresso (and there are plenty) would be clutching at straws to lower a 12 bar pump pressure to 10, expecting to see a significant improvement in their coffee, if they are not properly handling, and have a good understanding all the other vatriables involved.
So.....if you were on top of your coffee making, lowering the pump pressure on whatever your machine is, may give you a noticeable (to you) difference, possibly a good one.
Otherwise.....if you are happy bwith your machine and what it is doing for you with your current coffee making capabilty, it may not make much difference (to you).
There are an aweful lot of people in these forums, drinking specifications , brand names and labels. If you blind folded them and did a one on one comparison between a machine set to 12 bar, and an identical machine set to 10 bars......they may or may not pick any differences and if they did, could they pin them on only the effect of the difference in pump pressure?
Reading all this stuff on the www, opening up all the opportunities for difference, only seems to complicate matters for some.
Only you will know if you try.
To complicate all the above,* some machines dont have a means to adjust the pump bypass pressure from the factory set point, and if they work, they just work!
Hope that helps.
Attilio aka Billy Big Potatoes* ;)
Coffee and Coffee Equipment Psychologist, and
very first CS site sponsor.