If you want a real upgrade, my recommendation would be to upgrade not only the derailleur, but also the cassette and shifter to have a more modern system (transmission for single chainring systems, with clutch derailleur).
The reason is quite simple: 8-speed systems are now only entry-level, and are often used with triple chainrings. Cassettes have then very small range, and it's not by upgrading only the derailleur that you'll have a significantly better experience: I don't know about 8-speed Deore XT for example, but 9-speed Deore XT could only handle 11-34 cassettes, which is not significantly different to what you have (260% range vs 300%) - it will feel better for sure, but won't be functionnally much better.
By contrast, if you switch to a modern 1x system, you'll have more range and thanks to the clutch derailleur, you'll have less chain drops over rough terrain. That will significantly improve your bike.
At the time of writing of this answer, the "starting" configurations for modern systems are:
- Microshift Advent: 9-speed, 11/42 or 11/46 cassettes (380%/420%).
- Microshift Advent-X: 10-speed, 11/48 cassettes
- Shimano Deore M5120: 10-speed, 11/46 cassettes
- Shimano Deore M5100: 11-speed, 11/51 cassettes
- SRAM NX Eagle: 12-speed, 11/50 cassettes
In 12-speeds, things are a bit more complicated: because cassettes very often require another freehub body (to be able to use smaller sprockets than 11), which often require to replace the hub and then the wheel.