1gig plan giving 400mbps speed

Hi. Newish to Xfinity and recently upgraded plan from 400mbps to 1gig. Xfinity is supposedly delivering 1gig service but my wifi speeds are consistently 300-450mbps according to multiple speed test sites. I'm using Xfinity Gateway XB7 and TPLink mesh wifi which is capable of 1gig. All equipment has been power cycled and rebooted multiple times. Appreciate your assistance.