Hmmm. I\'m never really sure tbh! You read all the bumf on the subject of brake fluid and it says "oooh, never use this type on the street because it\'ll go off unless you change it" and then meet someone who\'s used it for ages and never had problems with it. To be fair, mate, you probably know a lot more about it than me! Your car sounds like a truly well-sorted beast, and if you happily use dot 5.1 on the street, that\'s good enough for me.
I\'ve not had a chance to get into playing with the brake system on my FD properly yet (there\'s always been something more pressing urgent to fix!) but I have put a lot of time and money into faffing with braking systems on my bikes over the years. The best combination I ever found was ductile iron rotors mounted on a circlip drive rather than bobbins, fully-floating on the bell and using an organic rather than a sintered compound of pad. I used this set-up with proper Silkolene racing spec brake fluid and with bog-stock off the shelf Halfords unbranded motor factor fluid, and it seemed to make no difference! Like I said, the major overriding factor seems to be more how much heat the pads/disc combination can lose without transferring it to the fluid in the first place.
The problem\'s even further complicated by the silicone-based fluids, which allegedly don\'t absorb water at all like the glycol or whatever they are traditional type, but still operate under the same DOT ratings! I think it\'s something that should be really straightforward (isn\'t that the point of a universal rating system?) but in actuality is a bit of a black art. I guess the bottom line is that for street use you\'re seldom going to stretch even the standard capabilities of the FD\'s brake system, it\'s only like you said in the first place that when you\'re using it for heavy track work that the cracks will start to show.