Do the Spec shoes stretch with heat/moisture, perhaps that's what you were feeling on your rides with them.
i never noticed any stretch with my SIDIs or with my specialized shoes. feet also tend to swell with heat/pressure, so any stretch might be masked. either way, this is not something that ever came up for me.
With regards to the Sidis, did you run the stock insoles or aftermarket insoles?
aftermarket. i used one molded to my foot.
Maybe the solution is to try the new Spec shoes with the older/thicker insoles to take up volume.
i personally wouldn't use insoles to fix a fit issue. if a particular brand of shoe is too high volume for me, i would find something else that more closely matches the profile of my foot. (this is true for ski boots--lots of ways to take up volume but best to start with something that fits closest.)
for me, the purpose of the footbed is to provide a solid connection between the bottom of my foot (contoured) and the shoe (flat).
if taking up volume is your issue, you can always go out and buy some bontex. i just don't think the footbed is going to take up room in the forefoot the way you hope.
out of curiosity, why does a little extra volume in the forefoot (you mean toe box?) matter? for me, the precision of the fit is across the instep and in the heel. my toes are relaxed--a tight fit there doesn't serve me. just my own opinion, but i wonder if you can share your rationale to help me understand.