When my 756P3 came on workbench to LNA modification, it was the time to test 7300 on EME signals.
Not digging deeper into RX architecture and differences, test show that both receivers performs well and 7300 without LNA is usefull for EME try, even with single antenna.
At the time I can't say 7300 is better than 756P3, because having both connected I can't see much difference.
IC-756P3 RX screen:
Maybe (only my subjective look) 756P3 with internal preamps is 1-2dBJT better at EME decodes.
IC-7300 RX screen:
On the other hand 7300 seems to be a little "quieter" at the same time - maybe because of (as per Icom adverisement ) extremely low RX phase noise.
At least I can say having 7300 as spare RIG for 6m is good idea.