The P-51D-15 has been mentioned several times here as a incoming P-51D block.
I read that USN/MARINES pilots in case of Corsairs weapons preferred the greater ammunition capacity of the machine guns. Finaly nearly 50 F4U-1C were upgraded to F4U-1D with 0.5cal MGs... They admitted cannons bigger effectiveness especially against ground targets, but still more ammo was more ammo.
Do not forget that US pilots did not fight against 4-engines bomber armada or against extremely armoured Il-2s. 0.5 cal MGs were enough against fighters and 2-engines bombers and as a bonus allowed longer time of fire than cannons equiped planes.
About "noticable peformance differences between the blocks", i do not think so.
The certain difference was between D-5 and rest of D models because of missing dorsal fin fillet.
And may be between Mustangs operating from IwoJima and rest beacause of battery movement forward. At least one pilot from 7th AF commented that Mustang after this was slightly manoeuvrable.
The biggest impact on plane performance was due to fuel, not plane block.