Well as I said in another post, based on this statement
"He could cut his foe so quickly that blood would not even stick to the blade. Feeling that he had reached the realm of the gods, he named that technique 'ÒÇî'God Flash'ÒÇì'."
Volume 3, chapter 2, part 2
Perhaps it is possible to obtain Brain speed if we could calculate the necessary speed for a katana to, i think, overcome the dynamic viscosity of blood.
I don't know if it is possible.
Speaking about necessary speed to let heat trails, I found some sites talking about ram pressure for meteorites and, for supersonic and hypersonic aircraft, i found the term "aerodynamic heating".
The mathematics behind it seem "heavy" and the problem is that we need, i beleive, a time and a temperature. For example, if we assume that the necessary heat for an object to start to leave heat trails is 1500 ┬║C and that the feat was performed in 5 seconds, we should be able to calculate the necessary speed that an object need to reach to get said temperature in 5 seconds.
In the literature that I found at least, they seem to speak about variations of temperature in regard to time.
However, we would be making major assumptions because this calculations are for objects with a different shape than Shalltear.
Duarte, G., Silva, M., & Castro, B. D. M. (2009, November). Aerodynamic Heating of Missile/Rocket-Conceptual Design Phase. In Proceeding of COBEM (pp. 15-20).
Hsiao, K. L. Calculation Aerodynamic Heating to A Stagnation Point of A High Speed 2-D Cylindrical Nose for Protection the Mechatronics System and Structure.