I might as well post the History of the Shillelagh document from DTIC, it gives some more details on the Shillelagh II.
The interesting point is that it was made for the XM-803, implying that it may have switched fully to the 105mm M68 if the program continued. The 105 may have been chosen either because there were still problems with the 152mm gun or cartridge, because the 105 was more efficient overall or because the 105 was already in service and would reduce cost and risks, which makes sense for the XM-803. Range was also boosted to 4000m.
I wouldn't rule out penetration being close to the 152mm warhead due to the greater built-in standoff. It didn't really matter anyway since the US didn't look into proper warheads against composite armor until 1977.
XM1 didn't use a missile but that might have been more due to the extremely stringent cost limitation and austere design. The US still designed significant reliability PIPs for Shillelagh as late as 1980, including a laser guided version which were cancelled when Abrams decided to retire Sheridan and M60A2.
