I think with diesel P&G could be a good idea but someone with the specialized testing equipment would need to measure it. Sadly disabling EGR on a diesel gives an increase in fuel economy but a huge increase in NOX emissions. I don't think saving a litre or so of diesel is worth a few extra hundred tons of emissions per year
With petrol I have only tested engine off glide and found that while it improved fuel economy it gave almost 4x the pollution output. I wasn't even able to get accurate measures because on restart it pegged my 5 gas analyzer at max
-Michael