I think that the important difference between fast and slow acceleration is the time spent at lower speeds. If your goal is to get to 100km/h then the most efficient way to get there is to religiously follow the BSFC to keep your engine running efficiently, this means you can glide for a longer proportion of your driving and you get great FE. On the other hand if you accelerate slowly then the time you spend at low (efficient) speeds where aero and rolling resistance losses are smaller is greater. Since aero losses increase cubically with velocity the lower drag from the lower average speed can make up for the decreased engine efficiency (i.e. you are getting less power per kg of fuel but still using less fuel over all because the power requirement is less).
I guess if you prefer a simple rule without having to work to hard to get decent FE.
If you hate slow acceleration and are willing to work harder (watching throttle and rpm carefully, possibly EOCing or P&G) then it may be better to accelerate with the BSFC map.
Probably the best thing for each driver to do is to test both methods and see which one works best for him/her. (or downsize your engine so you get high load/slow acceleration!
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