I think that the conversion fee is superfluous. The interest paid on HBD savings could be a factor that drives demand for Hive but the 5% fee effectively cuts into the APR right off the bat. There is already an inherent risk that the conversion will result in receiving less HBD per hive due to the 3.5-day delay in completing the operation (compared to the market rate).

Also, if the conversion operation uses the lowest price from the feed as initially proposed by @blocktrades (instead of the median), we have another hidden cost under certain market conditions. But I am not sure if that logic was left in the code.