Yes, the MOD which replaced the memory modules was done mostly for testing purpose.
But nothing is stopping the team to benchmark the modded card further in games and other graphic intensive apps, apart from just running the Unigine Superposition benchmark.
Would the RTX 4090 driver be able to utilize the faster memory?
I'm not sure if I understand this correctly. Are you asking whether you can use the regular GPU drivers of the 4090 on the Modded GPU ?
Because the regular GPU drivers won't be able to work and detect the GPU properly.
The team first had to make few other changes to get everything to work, and these included modifying the strap settings, BIOS, and changing the power supply project from 1.8V to 1.2V (External).
Once done, the 551.86 version driver was able to recognize the modded GPU with faster memory.