You are trading a lot of speed for retrieval.
The current theory is that retrieval is really useful for getting the common stuff (crafting materials, lower lv. materia, Company Crafting unlock stuff) but it doesn't seem to affect the rare retrieval yields.
If you are mostly after the rare drops the preferred builds will get to make twice as many trips as yours and will probably find a lot more rares.
If you want to go slow, you need to increase the range so you can hit more zones in one trip. Ideally twice as many(or more) zones in twice the time (or less) as the faster ships.
I don't know if anyone else has done any research here on fast vs slow boats, but I think the most important factor would be how often you will check them. If you are only going to check them once a week or so go slow, if you are going to be playing a lot fast will probably net you slightly more loot.
