The Me262 in numbers over Normandy would have definitely changed the outcome of that corner of WW2, I think. The only reason the allied armies on the ground in France could hold on in the beginning was air supremacy, including the ability to not only protect protect their own armor but attack the German. The Germans couldn't even move in daylight because of this, so despite their superior armor they eventually were driven back.
Lots of Me262s would have (a) slowed the Typhoon and P-51 attacks and, as plan_D points out, (b) play havoc with the allied ground forces. Not to mention they might have cut off the naval supply lines over the channel.
I don't think it would have won the war, but had it been built in numbers in '43, it would stopped the immediate threat of a Western Front invasion in 1944, I reckon.