Does Epson still make the V600?
If you are mainly scanning flats, the V800/850 are honestly overkill IMO. Their real strength and value is their ability to scan large film(up to 8x10), and I'd dare say that for the typical buyer of this scanner, the ability to scan essentially every film format currently in existence is the selling point and flats at best are secondary.
With that said, I have a V700, which is the predecessor model to the V800/V850. There again, I bought it primarily to scan medium and large format film, although now it only does flats and large format for me(as I have a dedicated medium format film scanner).
I don't find it particularly slow on flats at 300ppi, and I've found it's pretty unusual to be able to get more detail from a typical print than you can scanning at that resolution.