We are about the same size (173#) and I race. My boots are "set" for 115 flex but then I crank the booster strap down so the top buckle loosens. I'm not sure that increases the flex or not. At most, they might be 120. Last year I tried my boots with the setting around 135. The L2's and L3's I was working with noted that I wasn't as forward and it looked like my ankle wasn't closing. At the time I was probably 165# and not skiing as well as this year. I didn't increase the forward lean when I increased the flex.
My reason for dropping the flex back down was I lacked the force to flex the boot enough to bend the ski and it put me in the back seat. Could a more talented person the same size use a stiffer boot; without a doubt. I race with a friend that is very similar in size to me and has race boots that are 130's. He's also been skiing over 50 years. The forward lean in his boots is more forward than mine. I also work with someone that is close to 30# lighter and has boots that are 130 flex. Both skiers are very good racers, very technical skiers with a long skiing history.
I guess you can use any stiffness boot the allows you to pressure the front of the ski and doesn't push you into the backseat.
I think, especially with normal sized people like us , if the flex is increased, something has to be done to shorten the dorsi-flexion angle (more forward lean or toe lifts). Maybe its a matter of strengthening whatever muscle it is that helps you flex (I know body position is part of it).
I think height and not just weight play into this also (this gets into the whole argument on binding release settings and which is more important - height or weight). Height of the boot is going to affect your ability to flex too.
"How stiff is too stiff for a 175 lb skier who skis fast with good technique."
From asking similar questions in the past, I received and answer along the lines of "you should be able to get your knees over your toes when you flex".
Maybe a way to check is don your current boots and skis, place a scale under the tip of your skis and see how much pressure (measured in weight) you put on the tip. As long as a higher flex boot lets you do the same, you should be OK.
The boot also needs to be robust enough for the demands you are going to put on it.
I think boot flex, especially among men, is another way of bragging (Mines bigger!). Think of the drills more experienced skiers do - ski with boots unbuckles. You need to be well balanced to ski well with your boots unbuckled.
3 weeks ago I had one of my scariest nastar runs. Had trouble getting around a couple gates and just barely cleared a couple. It was as if my skis weren't listening to me. It was a fairly straight course too. When I went inside I realized I never re-buckled my boots before I raced . I had un-buckled them to set fencing at the start area. Top and middle buckles (Kryptons) were wide open. Re-buckled and ran the course - skis listened.
So the Goldy Locks rule applies. Too soft isn't good, too stiff isn't good. They need to be "just right."