It can depend on the orthotic and where the support is...if the orthotic is filling a void in your foot area then you may be fine...Sorry to hear about it..is it foot pain you are having or something up river more caused by the foot?
Personally I think it is the steel shank causing your problems .......clearly joking :)
Sounds like I have a high arches that needs proper support, it is collapsing a bit and causing the onset of a bunion on one foot. I am not in pain but noticed that I could feel some discomfort from the bunion area rubbing. From what I understand is that mine is very minor and the use of the orthotics at this point could stop anything from progressing or possibly even correct the issue.
I wear insoles in all my boots/shoes. I don't have custom orthotics but use insoles because I lack enough padding on my soles so the hard insoles of (mostly) boots are painful. As a result, I do usually size up 1/2.
I wonder if you could ask your podiatrist or whoever is making the orthotics if it is possible to make the orthotics so that it will have minimal effect on the "volume" displaced? Arch support might nevertheless make a difference to instep tightness but maybe a "low volume" orthotic might mitigate other problems? FWIW, I have no clue what I'm talking about, just throwing out ideas.