That is though exactly like i want them to be. So i will not change it, the moment you increase either you will also find yourself with a supersoldier according to my tests. They can get such incredible high defenses and armour it was not even funny - in fact it was fun as hell. So the lowish aim and mobility is there to keep their power in check. Mobility will always go down on promotion but on the other hand they get very early armour and plenty. Aim on the other hand is a bit random. Can go down or up

You may want to rework the stat growth for this. In my last campaign both of the soldiers who got this as a primary spec had their mobility go down after promotion. Combined with the poor aim growth they were pretty much dead weight on most missions.