Is this even possible?

Wetsuits can easily take 10-20 sec per hundred off for me. 

Not sure if it an take off quite as much as you found, but maybe today was your breakthrough day.

Don't dismiss this so quickly. If you were doing work that improved your form, it's very possible. The suit probably kept you more in to swim right as well. And, don't discount the possibility that you are simply gaining comfort in open water. Your earlier times probably reflect some discomfort.

I have no doubt that your form, fitness, and the wetsuit all played a major factor.  But here are two more factors to consider. 

1st How well/often are you sighting.  Was it really 2k you swam today and 2.5k last time because of the zig zagging?

2nd, is there any chance of current/tide having an effect.

I had thought about these factors as well:

1) Sighting - I can't say it hasn't improved.  I use prescription goggles.  My new pair (Aquasphere Eagles) are a lot better than my previous pair as they don't give me "tunnel vision".  That being said; my sighting previously wasn't that far off.  Plus previously I swam with friends who sighted better than me and I could just follow them for a large part.

2) Current - if there was a current helping in one direction it would have hindered me in the other I would think.  My splits going both ways was pretty much equal; so I can discount that being a factor.

I have little doubt that my swim fitness is better; what I'm surprised with is the fact that nothing that happened in the pool over the past two months has given me any indication of an improvement on this scale.

I am sure you saw some great efficiency improvements in body positioning, due to the suit, as well as the increased amount of time in the pool beforehand.  If I can stay consistent with my pool work before an OWS it typically translates into a good OWS performance.  I think I see this, and possibly you, do to how terrible I am before getting in a lot of laps beforehand.  I typically have started the year in the 2 min 45 sec/100 range and by April, after thousands of yards in the pool, I can pull off 1 min 55 seconds/100 in an OWS. 

I think sighting and wetsuit usage coupled with your increase pool work beforehand led to your improvement.  Great job!

That is great work and others have posted good possibilities.  Extra pool time does make a difference.

For the #2 item - Current, I wouldn't completely discount this one though.  While if you had a tail current/head current, those would cancel eachother out kinda like on a bike with a tail wind, assuming you did an out and back.  As you noted, your times were even in both directions.

If you had a current pushing hard from the side previously, and you did an out and back, you could have been "fighting" to stay straight in both directions.  If there was no cross-current with your recent swim, then it could have been "easier".  Possible?

Either way, I'd take the swim as a good sign.

Are you sure you didn't have a clock error?  That time difference is not something that a human could achieve with 2 months of work and a wetsuit.

Wetsuits help...but definately not that much.

Form training helps...but definately not that much.

100% improvement in that short of time is hard for me to picture.  (I guess you too)

Originally that was my assumption - my watch is off.  When I swam the first 1K in 15 minutes I figured "Must have hit the stop and start button somewhere back there ... it takes me nearly half an hour to get here usually".  I figured the trip back would be the equalizer - but the split was the same.  As soon as I was out I asked my wife (who was waiting on the shore for me) "Did I look faster?" to which she replied "well, you were out of sight within the first 5 minutes ... usually it takes about 10".

As for current; there was a mild side current (which you could tell from the ripples in the sand underwater) - nothing significant.  Pretty much the same sea conditions I swam in during the summer months (just colder).