Rene, I'm not a tire expert, only a (frequent) consumer, so take the following with a grain of salt. For several years in the late '80s we competed in autocross (1st & 2nd gear only) in a home-built Gr-3-type '72 pre-L with a high-power engine for the time and a pair of 245-50x 15 Yokohama radials on 8" front wheels with Hoosier 295-50 x 15 belted-bias tires on 10" wheels in back. We literally did not care if the treads matched all around- this was about competing at speed, not a car show.
The car handled well enough with "mismatched tires" to win a State Championship in B/Prepared Stock one year & as runner-up to a Corvette with much larger tires the next year. So I suspect the prohibition of mismatching tires is for those ON THE SAME AXLE, not necessarily the same on both ends of the car. In other words, one should not run a Yokohama on the left front and a Goodyear on the right front. Squirrely handling might then show up, especially at higher speeds than those we enjoyed with this Pantera.
Note also the soft, road-legal Hoosier DOTs in the rear were both worn to the cords in less than 1000 miles of autocrossing plus legal street driving to and from the 12 events that year... Your mileage may vary!