Yeah you can make adjustments to technique and you might gain the odd second here or there. But you get the volume up around the 15km mark and you’ll take huge chunks of time off, talking 10’s and 20’s per hundred.
Just my experience. I never got noticeably quicker with more technique work.
I also noticed my technique naturally improved significantly with the additional fitness, my form in the water was much better than I could ever achieve prior.
For the record I brought my swim times down from 3:00/100 to 1:12/100 over the sprint distance.