There are still some bugs to work out with my Open XML implementation, to be sure. However, it was taking about 80 to 90 seconds to open the entity with the 10,000 line report in the old code base. It's now taking 43 seconds. Of that 43 seconds, about 15 seconds is retrieving the big chunk of XML that describes the data values that need to be updated on the 10,000 line report. The actual update with Open XML takes 4 seconds.
This is looking promising if we can shake the bugs out. :)