At my current client we have reports in excess of 400k rows with 100+ columns. Normally this would cause DSP to have do to an application pool reset (at the very least) and also produce a flat file of the report instead of the excel format. We've managed to circumvent this behavior by allowing these "big" reports to be added to the Target Export page and having Cranport creating the excel files instead of the Transform process to create reports. We then modified My Reports slightly to allow users to access and download these reports. It works very well and other clients may be able to utilize this functionality or the framework could be updated to make this seamless.
Not sure if utilizing CranPort in certain instances is the way to go, but in general, Transform should be able to produce large reports without failing.