I am a tech but had to look up the names of the two seats you referenced. These are basically shield boosters - I didn't catch that from the description of "safety cushions".
There is only one shield booster currently being sold in the United States. There are people here using it (made by Kidde) and many threads on the topic.
I personally think the product being offered here in the states is not a good choice for most families. The weight limit on the shield is only 40 lbs. I think it would be a nice step in between a harnessed seat and a booster, but it should start at 40 lbs and not end at 40 lbs.
Britax has recently posted some interesting videos on the topic. While Britax is a clearly a biased source, it confirms my worries on this product. For a smaller user, there isn't enough protection to keep the child in the vehicle in the event of a rollover.
Do I think there are some limited situations where it might be an ok choice? Maybe a child who recentl had surgery that makes this a better choice than a 5 point harness.
I can't imagine the extreme percetiles mattering much. A very tall, heavy younger child would most likely still fit in an appropriate harnessed seat.
A very slender, think older child might fit in the shield booster, but I am not sure if it is necessary. My son is 5.5 and still isn't 40 lbs. Once he is 6, he will get to selectively use a turbo booster even if he is still under 40 lbs. There is no way I would pay for a Cybex for a 6 year old.