Getting about 510Mbps would be about right. The MR33 will do two spatial streams with a 80MHz channel width on 5GHz, this will give you a maximum PHY speed of 867Mbps (and that’s only if your close enough to the access point that the signal to noise ratio is good enough to achieve the highest speed). However even if you achieve the highest PHY rate the data throughput will be less (usually somewhere between 50% and 70% the PHY rate) due to slower devices on the same network, the overhead of the wireless protocols, and inefficiencies, to name a few.
The MR53 will add more spatial streams, but unless your device can make use of them it won’t increase your throughput, I believe the iPhone XS only supports 2 spatial streams, so it’s unlikely to improve much, if at all. With the 4x4 MIMO on the MR53 you may get a better signal to noise ratio further from the AP, so you may get the highest PHY rate further away, but that depends a lot on your environment.
Likewise, moving to a Wifi6 MR36 (which implements High Efficiency features) may bring some marginal benefits, but you’ll see the biggest jump if you also update your phone to one that supports Wifi6 too... maybe an iPhone 12 is in your future?
