Are there any downsides to the rear wheels being bigger than the fronts and/or rear axle being wider than the front? Any interesting internet reading pages much appreciated. Thanks.

Them McLaren and Ferrari F1 cars seem to manage ok with it.....

There has been a few threads on this subject and the consensus seems to be that it doesn't matter much

Cheers
Alex
PS
i would say that though as mine is different front and rear

I reckon the only problem is squeezing through tight gaps. You may squeeze though with the front of your car but get snagged on the rear arches. I have heard of some racers equalising the front and rear widths so its one less thing to worry about when overtaking.