Basically every kilo you save is more useful load and range. It's also a very conservative market. Most of the planes in the process of being certified, or already certified (like the Pipistrel trainer) are using batteries that are quite unimpressive in terms of wh/kg. That's because the technology is typically at least half a decade old by the time a plane gets through the certification process.
Manufacturers can't just switch battery supplier without triggering a lot of re-certification activity and most of them would have locked in their supplier many years ago; long before even starting the certification process.. It's their next models that are being designed now that would get something that is state of the art now. Like this battery. We won't see those in the market until 6-7 years from now.
The exception to this is probably going to be experimental aircraft. I think that should become a growth market pretty soon. A 500wh/kg battery of say 100kwh would weigh about 200 kilos, give or take. That should get you some usable range and be light enough to put in a small plane. And it would put a nice little dent in the cost of what used to be a 100$ hamburger flight. Fuel cost is high and planes use a lot of it. 5-10 gallons per hour typically. And combustion engine maintenance on planes is very expensive. All that goes away with battery electric. Servicing becomes a lot simpler, less moving parts that can break and the few remaining ones last a lot longer.
Irresistible for a lot of private pilots, I would imagine. This is a market that doesn't currently exist but I don't think it should take long for people to start experimenting with battery electric.