Calculation Speed Slow with Excel 2011 for MAC - how can it be improved?

I have been developing a large financial model covering 12 years at monthly resolution - Excel 2007 file size is about 10MB - with about 14,000 input variables.
it runs reasonably quickly on a 4 core 2.3GHz desktop (Win7, plenty of RAM) and a 2 core 1.9Ghz laptop (Vista, 2GB RAM). However, one user is running it under Excel 2011 on a quite powerful Mac (he says it is the i5 processor). After a couple of significant additions to the model, the performance on the Mac became awful and it is now only usable if automatic calculation is turned off. (The PC version is fine.)

Excel 2007 uses as many cores as made available and has a larger dependency list so that only affected cells are recalcuated when a value change is made. Also, I understand that in earlier versions of Excel, the permitted maximum dependency list size was smaller and if it exceeded the limit, Excel started doing full recalculations.

From reading about it, Excel 2011 for Mac appears to be single-threaded. However, could this degredation of calculation speed on the Mac be due to the calculation dependency list getting too big and thus ignored?

I have removed all volatile functions (there were some OFFSETs used to allow variable range summation) but still have a few conditionally formatted cells which may behave as volatile. There are a lot of SUMIF functions used, for example to summarise monthly P&L data into years or select the year-end balance sheet values based on month and year indicator rows. Hard-coding the formulae to reduce SUMIF down to SUM would be a chore.

Are there any Mac experts out there with suggestions for things that can improve the speed?

Can you share a version that exhibits the problem but does not contain any sensitive data?