Of course I got thoughts...http://www.arhy.org/swim-predict
Fill in the original calculation sheet.
If your SDI is flatter than 1.06, then you may simply lack pure speed.
Working on catchup, as far as I am concerned, it not a sin at all. There are ways smarter than others that said.
Using a tempo trainer, time trial at low rate. If 60 is your sweet spot, go slightly below 50. And swim as fast as you can for 600m with a pull buoy. Monitor improvement from time to time.
I'd expect that you could go down close to 10min flat, given your best effort over 100m. If you fail at making it within 11:30, then yeah you could benefit from this sort of work a bit.
Not saying making an obsession, but once in a whilst.
What this site and most other serious sites would warn you against, is developing a stroke that is such that you can not do otherwise. If you can't swim in any other way than overgliding, then you're an overglider.
Several top level sprinters like to overglide at training during some long aerobic sets.
They look as follow: http://www.youtube.com/watch?v=AmRVoVKx3Hk
This is an extreme case, but it's not at all unusual. When's time to perform threshold stuff though, that guy will stop overgliding and start swimming. That's how I see it.