I don't disagree that pull bouys have their place. However, sinking legs and not being streamlined as a result will slow you down far more than any flaws in your stroke.
Best advice I got was to concentrate on balance drills. Within a month I was swimming so much easier and more efficiently...
Most people's legs don't really propel them much in the front crawl; they will however cause a lot of drag if you don't lift them up in line them up with the front of your body. The pull buoy will help lift them to an extent, so if you suffered from sinking legs, then you should go faster...
The pull buoy will only help while you're using it though; it's not going to correct a technique flaw, which is likely to be related to head position and overall balance. If anything, becoming too used to it will worsen the sinky legs and you'll be dependent on it.
Have you tried the Total Immersion books/DVD and the tekkers it teaches? I had been swimming for about 2 years, and had completed 750 swims within sprint Tris, but would've been too knackered to swim much further (especially without a wet suit), mainly because I had to kick my legs frantically...