I was also wondering that... it'd be nice if they did, and made it retroactive. I didn't know about the Google Ads link that JonoW posted, I'll have to look into that!

You just get double the space per referral, but you can not exceed the 18GB limit. That means you can get to 18GB faster, with fewer referrals.

But I don't know if it works retroactively for refs you already have.