+1 on the comments about needing to get them up to operating temperature etc. They are hard work to get up there, and at any temperatures below optimal, they are going to grip less than normal street tyres, with more grooves (part of the reason the street tyres warm up faster).
The ex-race tyre I did try using lasted such a short while (1500km) it cost the same as a new tyre, because I could get more km out of the new ones. That was running the race tyre almost bald, so it had a full life.
I wouldn't bother now, yes its legal, but grip levels etc...
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