Could well be the problem - I noticed quite a few aches and strains when moving from a 12mm to 6mm drop. Having said that, now I've adjusted I far prefer the lower drop - seems to promote a much smoother, lower impact and more efficient running style.
As far as I'm aware, Saucony are pretty much on their own with their reduction In drop across their range. I think New Balance may have done it on some shoes too, but other than that I think most manufacturers still have a normal drop on their running shoes.
I've just gone the other way - from a "normal" 12mm to 6mm drop. I didn't anticipate any difference (what can 6mm do ?) but was quite surprised.
I noticed that there was a slight adjustment period - nothing huge, but my back and calves ached more than usual for a couple of weeks.
The main difference though was that I run more smoothly with the lower drop - less of a heel impact/strike and I found I naturally tended to run a little faster. I don't know if the reverse would be true - more impact and slower if I went back to my old shoes, but going to a lower drop has certainly been positive for me.
If you have issues, there are lots of trail shoes with a lower heel. Inov-8 for example do a range of different heights.
I don't think there are any hard rules - shoes will last different times for different people depending on weight, running style etc.
Like you, I change mine every 300 miles whether they are obviously worn out or not and for the same reasons as you. I'd rather lose the £10-20 worth of running that may be still in them than get injured.