Do you have data about increase of slew rate with higher speed grades? Never found anything in literature.
I doubt any such will be available bacause, in many cases, the manufacturerer will want to - when needed - labe chips with the speed grade that is ordered regardless of the performance of the chip.
however I do recall that once when a chip became available in higher speed, the datasheet covering all speeds showed up with smaller Tmin for some parametres.

Thus it is likely that chips rated for a higher speed will be in the opposite end of the tolerance that chips rated for a lower speed.

Again, speed grade change should not be a problem
>>If your code is "designed to work", not "put togetheter and tested"<<


Erik