Just an FYI the opening and processing is probably much more closely tied to the subsystems of the associated systems than the CPU itself. The i7 is likely going to have a faster bus and be able to pull the pull the data off of storage faster than the older Xeon. I'm curious if you know if the machines are are comparing are running the same chip-set and storage type. E.g. Intel X79 Express and SATA 2 or something else.

And as mentioned above, as Excel isn't multi-thread aware a Xenon isn't the best choice for it either.