I was involved in a small project where we converted a spreadsheet model using Solver to GAMS/CONOPT. I was unable to reproduce the spreadsheet results, and it turns out the spreadsheet had some errors in the formula’s. I have converted quite a number of Excel models, and almost all had some problems that were uncovered by moving the model to a modeling language environment. See table 1 in http://panko.shidler.hawaii.edu/SSR/Mypapers/whatknow.htm for more evidence of how difficult it is to create error-free spreadsheet models.
PS: after fixing the spreadsheet errors we were able to get similar results. This is actually not a bad approach to verify and repair spreadsheet models.