LibreOffice and OpenOffice could scale you to the moon. Headless Linux-Mac-Win.
You can get the contents of spreadsheet with about 200 lines of code.
Use the OpenOffice.org (or LibreOffice) APIs to open the spreadsheet files, access the data, perform the transformations, and then write the proper data into the new database.
As long as your are not trying to do fancy things like inserting and updating formulas it's very simple and cross platform and because it's open source you can rack as many instances as you need. It opens up xls files by converting them to oxd. In Linux you can run in headless mode and if you multi procs you can startup multi OpenOffice daemons to process multi docs at once. It's not marketed much as the "pro" solution, many instantly frown down on it. But I spent many years working on it starting from 2003 building a platform that printed millions impressions a year and 90% of the code is still running.
We started the implemention with 1.0 and as of last year they are using 3.x version. We didn't have to upgrade any code to adjust for the upgrade. You don't need any other XML API that's version-dependent you won't need to worry about comparability Open Office 3.x will open 97-2007 xls. I recommend this because I was a Platfrom R&D analyst and Java dev. for 8 years and I spent better part of three years researching this solution agents others.