I've tried a couple of flavours of performance diesel, and any improvements in economy have been far too marginal to justify the increase in cost. If I can stick to a steady 70 in top gear and cruise control on a good run of motorway (around 100 miles or more) then it should give a pretty good indication.
Here's what I'm getting:
BP Regular diesel: 53mpg
BP Ultimate diesel: 53mpg
Total Excellium: 53mpg
The resolution on the Lag's trip computer isn't the best, but it would certainly show up the 5% difference required to make up for the extra cost. There is the variable of headwind (there's not much I can do about that) but I even check on the GPS to make sure that I start and end the measurement at similar altitudes (believe it or not, a difference of a couple of hundred feet can show on your economy figures, even if you've travelled over 50 miles.)
I have found much the same as you Horatio over brim to brim fills.
I think you would save more mpg over a year keeping your windows closed, removing roof racks etc.
some I know swear by the improvement in their cars when run on the Premium fuels and are positive when they have changed back their cars do not run properly.
As I have mentioned earlier I have tried them in a variety of cars ( Diesel & Petrol )and have not noticed any real difference in mpg or smoothness in the cars performance.
Maybe it's a bit like snake
if you believe it is making a difference ?