If you want a no-hassle no-modifications upgrade, get a Traxxas 2085x. It drops right in and provided a mild speed and torque increase.

If you want an actual substantial upgrade, then you have to accept that you may have to do a bit of trimming to get a good servo to fit right. I'm not 100% sure about the XRT but I think that's required in the Maxx and X-Maxx so it's probably necessary on the XRT too. If you go this route you'll need:

A servo adapter because the stock servo is a proprietary size. Traxxas makes a plastic one or Hot Racing makes an aluminum one.

A good servo. Get something with a full metal casing for heat dissipation and that accepts up to 8.4V in case you want to add a BEC for more power and speed later. This one from AGFRC is commonly recommended here. and will provide a little more speed and torque at 6V than the Traxxas 2085x OEM upgrade servo I linked above. If you want even more speed and torque, get...

A BEC. It's not strictly necessary, but your stock ESC's BEC can only do 6 volts. To get extra power and speed from your servo, get a separate BEC that can do 8.4V. That'll give you about double the torque and 1/3 faster speed from the AGFRC servo I linked, compared to the stock XRT servo. You can start out without a separate BEC and if you're still not satisfied then install one.