Gear pumps are not terrible on turret work, just not quite as good as centrifugal. I would say with most gear pumps you could get 80' - 100' from a turret. Gear pumps run a maximum of 400 RPM so perhaps that is part of it. When it comes to distance, flow can sometimes be as important or more important than pressure and centrifugal pumps have a lot more flow.
I am a little surprised you have to thin down your slurry to pump 150'. May I ask what size of hose you are running? I can't help with the Finn part. I could make an educated guess and probably be right on but the guys using them don't have to guess so I will leave that part of your question to them.
Gear pumps are easy to work on so it is not something that anyone with minimal mechanical ability needs to be afraid of.