The Allison engined models were great aircraft but so were all the other low altitude planes, it's above 20,000ft were the men were seperated from the boys and where the air war ultimately progressed.
I am sure these gentlemen, who flew their war primarily between zero feet and 8,000ft between 1939 and 1945 would disagree with you. As would their DSOs with Bars (multiple), DFCs with Bars (multiple), M-i-Ds (multiple), plus numerous other decorations would indicate that the war below 20,000ft in the face of intense anti aircraft fire from light to heavy AA fire that could reach them in that envelope, usually being outnumbered by enemy fighter bouncing' them from a great height whilst "down on the deck", would beg to differ.
A number of those pictured did also perform operations in the "pure" world of fighter operations above 20,000ft, but still recalled their low level operations at zero feet as the ones that required the most 'nerve' where the threat came from not just the enemy but power and telephone lines, church steeples, chimneys, wireless towers, large seabirds (a couple pictured returned from sorties with their aircraft badly damaged from impacting large flocks of seagulls as the crossed the Channel at zero feet), along with sea mist that destroyed any semblance of a horizon whilst flying mere feet above the sea.
One of those pictured here, returned from a low level sortie to obtain photography of a Gestapo headquarters in occupied France with bullet holes in his aircraft received from enemy AA, that could only have been made by the enemy AA gunner firing DOWN upon him - one of his photos from the sortie captured through the window of an upper floor of the enemy HQ, the light fitting on the ceiling of the room captured though the window. Without the low level sorties flown by these pilots, from 1939 to 1945, the War may have turned out a bit differently.
And yes, they did fly low, and they relied on the Allison engines in their Mustangs to get them there and back, from mid-1942 until August 1945.