OST flamer upgrade competes for muni with healing, mg42s and g43s for grens, while flamer for SOV is decent damage boost for con openings + it competes only with mines early game. While higher vet does not provide a lot of combat bonuses for combat engies, it reduced downtime for t70 later a lot (+60% repair speed for engies with vet 2). Take a look at top players that rarely skip flamer upgrade with SOV and even go with 2 flamers vs particular builds, on the contrary it is ok for OST to skip flamer all together.
Yes CE vet easier and they will continue to vet easier if XP value of both unit is increased proportionately.
In sort increase XP value will have the same affect for both factions.