Swimming improvement - frequency or duration

I'm certainly no swimming expert, but I think increasing your distance and frequency will help.  Two examples:

1) This year, I went from a 2:11/100m pace to a 1:52/100m pace by swimming 2000-3000m workouts, 3-4 times a week.  In my olympic distance races, this resulted in a savings of almost 5 minutes (32:49 to 28:05).  I think that the tri club/master's swims (intervals) over the winter helped, but the long open water swims seemed to show me the most improvement. 

2) A friend of mine did a 25:17 1500m OW race (1:41/100m) last July.  Over the next year, he swam 4-5 times a week, doing distances of at least 2000m each workout - intervals in the pool over the winter, switching to mostly open water swims in the spring.  This July, he did the same race in 22:57 (1:31/100m).  He had to work a lot harder to reduce his pace than I did, because he started out at a much faster pace to begin with.

Anyway, I think that anyone's swim times would benefit from increased frequency and duration.  If you can do a swim or two with master's, and at least one long steady swim (or two) each week, you'll come out ahead.

Not sure if I'm following the math here.  Isn't a savings of 15 seconds/100m over 3800m (Ironman distance) equal to 9.5 minutes?

winning AG with those time? where, cuz certainly not a big/competitive races? He/she needs speed and endurance with those times? spending time in the water is a waste cuz droping his/her avg 15 sec would only yield 6 min saving over an IM distance? what about his bigger aerobic base? what about the little energy spend in the 1st leg of the race and extr energy availabe for bike/run? *sigh*

Here's the typical time range where you would have to be to come out of the water in the first few people of the AG at a competitive race:

Oly. - 20 - 22 minutes (sub 20 in a really strong field)

HIM - 27 - 30 minutes

IM - 50 - 55 minutes (sub 50 in a really strong field)

Those are all in the 1:10 - 1:20 range .......

If it were me knowing that I could maintain 1:40/100m I would jsut make sure that I could maintain that level and focus on my other limiters whether they were the bike or the run.  With the times you already put out you are coming out of the water in the upper portion of the field.  The goal should be to sustain that placing and improve upon it with the bike and run.

First out of the water rarely means first to finish ..... (although I'd kill for your split times).

I've always heard that speed sets will make you faster.  5x200, 10x100, 10x50...  Certainly you will gain sprint strength.  Combine that with sessions that focus on distance and technique, that's a formula that will get your times down.  My personal experience is that my swim times improve if I can swim 3x to 4x per week.  2x will maintain.  When I was swimming lots and doing a weekly sprint set like above for several weeks, I was able to do a set of 1:20-1:25 / 100m pace on a 2:00 interval.  I could probably sustain a ~1:35/100m pace on a 1500m swim at that point.  That was down from about a 1:45-1:50 pace previous to that.

As much as I've wanted to get my times down again in the pool, personally I stand to gain way more time in a race on the bike and run so I focus my training there.  Just maintaining with 2 good swims a week has me at a 25-minute short course mile, which works fine for me.

I remember swimming next to a gal once that was holding a 1:10/100m pace on long sets.  She's one of the fastest distance swimmers that I recall personally seeing, and she told me that she'd been a scholarship swimmer at some big university (Texas?).