It's notable that with the lifting of the displacement limit rule there is no longer the same kind of R&D going on by the leading engine manufacturers today. Granted they still aim at producing improved products, but there is really no need anymore to push performance to its limits when it's simply more practical to just bore out existing designs instead.

The primary goal today seems to be decreasing weight while increasing displacement. It's interesting to try and imagine what types of engines would be on the market today if the FAI had chosen to limit 2-stroke displacement to 1.20, for example, instead of dropping the limitation altogether.

As stiletto660 stated, the technological advancements that were the result of displacement limits made their way into sport engines over time. Today, the .46 has all but replaced the .40 and the .51/.52/.55 are in the process of replacing the .46s. The "just make it bigger" approach gives more power, it's true, but it does little in the way of driving technological advancements. On the other hand, maybe engine designers have gone as far as they can with glow technology, the same way full size prop-driven aircraft hit the speed wall. Bigger and lighter might be the only open path available in terms of advancement. I don't know.