Yeah- a cure-all in a can... that'll work.
Keep the filters clean, oil fresh, octane high and drive gently.
Flash that ECU and watch the dealer reflash it back to stock when you go in with problems... or when you fail emissions testing. Bad idea.
I will nit-pick a previous post about 'can't get both HP and economy' as that isn't always true. There are many mods that CAN do both and are dyno proven but they don't apply to Rovers. For instance, on a 1975 Camaro with a 350 V8, installing headers and dual exhaust combined with a mild "RV" cam and Edelbrock manifold w/Quadrajet- HP went up significantly as well as the fuel economy (as long as you drive gently). The HP gain was about 35 or so- fuel economy improved by 5 or 6%. It was a college project my stepbrother worked on and all dyno proven. A strong engine doesn't work as hard in normal driving and can often return better economy. For isntance, the 8cyl Ford F150 gets better fuel economy than the 6cyl.