Every deviation we make from the ISO rating and the ISO processing standard to any other speed or process standard is a person specific speed known as an Exposure Index, an EI.
Everything from our metering methods to the accuracy of our thermometers to our choice of paper can affect our personal EI choices.
I'm going to hazard a guess that outside of those who actually do ISO testing for film manufacturers, few if any of us mimic the ISO standards in practice. The film speeds that people brag about "finding" are in fact EI's.
This doesn't mean these films don't or won't do a good job at an EI of 3200 or 400. Box ratings are just numbers.
Even Ilford and Kodak show us various EI's that are workable. This data sheet for Delta 400 is a great example, http://ilfordphoto.com/Webfiles/2010628953322222.pdf see page 3, the numbers in bold.
If Ilford had chosen Microphen as the ISO stand developer for Delta 400 the ISO rating may have been 500, if Perceptol maybe ISO 250.
They could, it would have a short shelf life.
None that I know of.
No. 800-1000, depends on developer, see Kodak tech pub.