The PTE series have the PTE exhaust housings which may make them work a bit better on the top end and still spool on the lower end. The design seems okay when you look at it but my limited experience says they don't spool quite as well altho' the difference is not substantial. I think the PTE51 really comes alive with a smaller diameter, 3000 rpm stall converter but some have gotten satisfactory results at 2800.
I think the PTE chart is pretty close to right with the 54 being a bit weaker than a 63E or 45. I see no reason to go from a 60 series to one of the 50's, myself. On the other hand, if one has a good converter, the 51 is a heck of a good street turbo--if one has the stall.