[b] I would think the ABS would compensate for differences in braking adhesion, if there is any, but maybe not. [/b]
I think not. ABS really will not do much here, except when braking on slippery roads, like ice and snow.
While it would appear from your description the AWD 4WD issues are not applicable, but mismatching tyres front and rear can cause handing problems, under emergency conditions on dry roads, even with ABS.