However for most users the chances of a large account coming along and upvoting their post will remain small, even if the shift in incentives does change behaviour

There is more to it than that. An enormous portion of the reward pool is being milked out by self-voting and vote selling. Even a partial counter of that will increase the value paid out in STEEM per vshare. That also offsets the lower vshares per rshares at the low level of the curve resulting in higher rate of STEEM payout for those fewer vshares at the low end, even if no large stakeholder comes along and votes.

There are too many moving parts to do a simple calculation of payout for a particular vote assuming all else is equal and expect that to be remotely valid. It isn't.