It seems engine tuning for the McLaren and a econobox are being viewed as the same, eg: both just tune for stoichiometric with different engine sizes... If you hammer the McLaren engine it creates so much heat based on hp/liter that many things would be different from a very softly tuned engine.
In general, very high performance engines tune very rich under load so as to never blow up the engine with a simple hiccup, there is a safety margin for any leaning-out event. To operate at extreme hp/temps things like spark plug "temp" and gap are spec'd differently. The camshaft has to be profiled to match it's intended operating range, typically no where near economy revs. Some engines literally use un-burnt fuel to cool down the cylinder walls and keep internal temps in check. Some of these reasons are why you can see a fireball shoot out the exhaust at times. And we haven't even gotten into things like valve overlap, parasitic drag running coolant through 2+ radiators and on and on.
Hence, even piddling along
for an entire trip would probably still result in lackluster fuel economy.