if you want a WMO specifically, just keep an eye out on used product sites. ebay is great for the USA. WMOs are still frequently put up, often cheap enough.
they sometimes get put up as just "microsoft mouse" or something similar. keep that in mind if you search for one
or if you just wanted to try the MLT04:
IMEs and IMOs are usually a bit more expensive, but it's not impossible to find good deals on them.
the OMB (optical mouse blue) pops up a bit less, but is also an option
tbh if there even was a difference between those mice, i bet it's (far) under 1ms and not noticeable.
i think the mx518 has less weight which could make a difference in feel, as well as different feet if you're using the 1600 version of the mx518.
and of course those mice have different (native) cpi steps.
edit: and i would always compare mice using the same polling rate, it can affect the feel significantly
To be fair, I'm relatively sure that it did something sketchy or Valve accidentally added a sig into the database that wasn't long enough and did collateral damage. It couldn't have been bytepatching because the integrity checks on game code are gone nowadays.
"Push your mouse to the limited with Overdrive. Legendary shape enhanced with premium martials."