Mario,
Personally, if 40 inch wheels are available, I'd find that compromise acceptable for a single model. As far as this lowering the tender? Well the difference would be half an O scale inch, or .010 of an real inch. That would be 1/100th of an inch (or communally called ten thousands). There are some very sharp eyes in the O scale modeling world, but I bet none of them would catch that! Functionally, you would have to compensate for that .010" at the coupler.