I'd say that this just proves once again, that the NEC 3550/3551/4550/4551 are looney scanners!
Each of these drives can have a different idea of which scanning speeds are useful and which are ludicrous, and there's no guarantee that there will be even a single useful scanning speed for PIE.
For my NEC 4551 the only useful scanning speeds for PIF are 1x and 12x with 1x being most reliable, but too slow for everyday use. On my drive 5x and 8x scans are totally useless for PIE and PIF, and 1x/12x scanning speeds are not at all comparable to other drives for PIE levels.
Megan3142, I suggest you stick with 8x scanning speed for your particular 3550 drive if that gives you useful scans, and ignore it if someone else tells you that "5x is the standard scanning speed for NEC drives so you must use that"!