The only problem I have encountered using a cover plate - was that the clearance of the hopping foot at full "hop" was decreased by about half, and it interfered with easy motion of the fabric, particularly over seams. Ticked me off enough I only did a bit before I switched over to the 301. On my 15 - the foot is about 3/16" off the bed at it's highest "hop" and with the cover plate I was using (think it was a Singer 328) it cut it down to under 1/8" and that wasn't enough.
Since I was doing it just to see how the machine would handle it, I wasn't willing to modify the foot to see if I could get more height out of it.