Well, it depends on the task. If you're doing a lot of R/W it will make a difference like transactional DB's would make use of the faster interface / bandwidth.
I've got a handful of different NVME drives sitting around and there are some nuances between them that show their face when doing certain things and only then. Some have a Phison controller on them, others a SanDisk controller, some proprietary options as well.
If I'm doing an extended copy of say 100GB of data the Phison's will drop off after the DRAM is exhausted. The fastest one though seems to be the SN770 but, the key to making it faster is not to do a bulk dump to the drive but drop several separate moves to it.
So, there's more in play than meets the eye on the cover of my statements above if you're doing certain things it might be the best idea to use the 990 as the OS / apps might benefit from the higher bandwidth. I would simply clone it with clonezilla and pop it into the primary position and see how it feels compared to the XPG. If it feels snappier then leave it there and wipe the XPG and give it a new UUID so it doesn't conflict with the 990 and pop it back in for additional storage.