And if you are using HD SC Setup, it will automatically select an interleave appropriate for that particular Mac. There are patched versions of it floating around for use with non-Apple branded drives.

Because your drive was originally formatted on the LC (or so I presume), the interleave is indeed wrong for the Plus. The good news is that you will be happily surprised with the performance boost you have coming. It will never be fast, but it will be much less slow.