My guess would be that you would see poor performance on the RAID-1 in the ways similar to having a 5400 RPM and and 10k RPM drive as the RAID-1 pair. or having 2 completely different disks (like in the old days of one disk saying it had 16 platters and another one saying it had 4.. IO to them is very different.) Ones always going to have different IO performance than the other so sometimes things would be fast and others really slow.
But that would be my guess. If its different let us know.