Have to disagree with using 5.1 however - that is intended for race systems with VERY short lifetime.
Yes - it has higher
DRY BP's - which make it attractive to the racer. That element alone is the major asset of a brake fluid - especially one under intense usage - like racing - where the brakes are extremely active.
BUT - that comes with a price - it is much more affected by water-absorption. That makes no difference to the racer, who is going to refresh for each session. But
not good for a street bike.
The BP is rated on fresh 'dry' fluid - when water is absorbed - even in tiny amounts, the BP is reduced drastically.
DOT4 is plenty for street bike - and even some DOT3 can have excellent specs than can even exceed some 4's, even with some water incorporated.
i.e. is fresh (dry) 5.1 better than fresh 3?
Absolutely!
In the real world, that is not the case and 'wet' DOT3 will actually have better BP's than 'wet' 5.1.
In this case just because racers use it does NOT make it better for your street bike.