Replacing hard drives on PE T430

I currently have 8 300gb 10k hard drives in a mirror array.  If I replace one at a time letting them sink up with a 15k speed after I put the last the last one in and sinked.  Will the speed now all go at the 15k.  I understand always works at the slowest speed drive in system. So guessing once all replaced will now work faster.   This way easier then down loading entire system .. replacing all drives then uploading to the new drives.