I don't know the bolt circle diameter on that crank, but it's probably either 130mm or 110, and either way you should be able to use a 39. You won't notice much difference, though--it will be a LITTLE lower, but not a LOT lower, and you'll probably increase the number of duplicated gears.
If you're planning to get strong enough to replace the triple with a double (which is another low-return move, IMO), why not just wait and save yourself the money and trouble? People aren't REALLY laughing because you have a third ring on there.
You can certainly change the middle ring to a 39T. The shifting might not be perfect, since the ramps on the 52 are made to work with a 42. FSA does sell different big rings, made to shift with 39 or 42 middle rings. I'd try just the middle ring first, rather than buy both.
The difference in the gear ratios is the same as one cog lower.
Another thing that's often not understood by owners of triple cranks is that the middle ring is in nearly the same position as the big ring on a double. IMO, the chainlines with the middle ring should be treated the same as the big ring on a double, meaning that the middle ring and next to largest cog is the most extreme that should be used (at least for any length of time). Like wise, the big ring and two largest cogs are off limits. On the bright side, the middle ring and smallest cog should be useable.
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