Although I didnt follow a strict 400m S-L approach, one of my winter seasons was to train purely as a 100/200m runner in order to bring my speed down (PBs 11.1/22.2/47.5) to make my 400m progress.
I did mainly weights and plyos in the GPP then moving onto short track work, hills and maintained aerobic work.
I started the season runnign ugly in 49.3 as I wanted to maintain the local champs title - but running 10.8 and 21.4 my speed had improved - my next series of 400s (I hadnt planned to do any but couldnt resist!) went - 47.6/46.9/46.5/46.4 and 44.7 relay - at the end of the season.
This showed that the development of SE on top of the speed realy moved me on. The following year we maintained the speed into the winter as i had finished off the season so fast and went on to impove down to 46.0 and 21.1.
Speed worked for me and I had alot less injuries during this time too.
That’s great. Speed is critical to lowering performances via the mechanism of speed reserve- as you can see. Of course, if you hadn’t competed at 400, you might not have realized what you were actually ready for!