Update:

> Just for the purpose of overview I compiled a list of the DMIPS/MHz
> per core figures for various PPCs in ascending order (AltiVec capability
> is generally ignored, obviously):
> [...]
> e5500: 3.0

The table on page #2 of the QorIQ AMP fact sheet PDF indicates that the e6500 core didn't improve in that regard (comparing e6500's per-thread performance to e5500's per-core performance). The only thing that makes me wonder is that on said page Freescale makes it sound as if the e5500 never existed:

"The 64-bit e6500 core [...] enables more than twice the performance improvement as well as twice the power efficiency over Freescale's previous generation e500mc core through enhancements to the core itself, the move to 28 nm process technology, as well as dual-threading capability."

I'd rather think it's the e5500 core they enhanced to realize the e6500 core and didn't start with enhancing the e500mc core all over again ;-)