The way I have written it is correct. I was explaining from the pool's point of view not the miner's.
If we hold the number of shifts constant and increase the shift size, the payout for each shift will remain the same. If the payout remains the same for each shift, an increase in the number of shares paid per shift will result in a decrease in the amount paid per share.
Unless I've made a silly mistake somewhere, the reward for each share is on average (B + fx) * Dp / Dn * (1 - f) where B = reward, fx = transaction fee, Dp = Pool difficulty, Dn = network difficulty, and f=pool fee.
No matter how DrH changes the shifts or shift sizes, the average reward per share will remain the same -- although each share might be paid smaller amounts multiple times.