Well, it seems that the G48/G43X magwell is wide enough to fit a doublestack steel magazine. And a company is capitalizing on this by releasing a flush fit 15rd steel mag for the G48/43X:
https://shieldarms.com/glock-43x-48-15-round-mag
And video of it in action:
https://www.instagram.com/p/BuhO76RnWsu/
And here's a photo from arfcom, demonstrating a 18rd beretta mag in the G48, showing how a double stack steel mag will fit:
Overall, this seems like a really "have your cake and eat it too" development. The capacity of a G19, with the slim easy to carry G48 platform.