Interestingly, the saffers went for the Roikat as a means of whacking the oppo. The HV 75mm is quite pokey and it does get through a T55 front glacis.
That said, Imho, upgunning AJAX beyond 40mm will only put it more deeply into "tank killer" territory which we all know is a daft idea for the poor buggers crewing it. The bigger the gun, the more it will get treated like a big boys toy.