The same question came up internally not so long ago...
Historically we have taken measures to extend HCL acceptance of "sibling" servers to qualified servers, when equivalence across the range of HCL servers is roughly level. This was not possible with the PE2650 though, nor the PE2550 before it.
The PE2650 is entirely a separate design from the PE2600 (as were the PE2550 and PE2500 before them). Disclosure from Dell Engineering confirmed this prompting the decision to select the PE2600 only.
Since the PE2650 would require an entire certification to be preformed we evaluated where it would fit in our portfolio. Because of its limited PCI space it could not support large TDM integrations. Therefore we made the decision not to support the PE2650 entirely and focus on other platforms which would better match the needs of the masses.
Thanks,
Keith