Do surveyors access the database very often? The critical question is not whether you have the max number of users for Filemaker Pro but what are they doing? What kind of scripts do you have? Do you use Replace Field on 50,000 records. How often do you sort and how many records.
I would suggest you look at the very fast tablets that run Filemaker Pro, not Go. These give MacBook speed but they are Windows. Last week or so I wrote a page or two on my Blog about this www.gofm.biz. Pictures and urls... Frankly if I were going to do a project that wanted to use something like an iPad, I'd give serious thought to using one of these new tablets instead. They are many, many factors faster than an iPad or iPhone.
You don't have to use Go, you know.