Generally speaking, yes, overfilling isn't a good idea, but there's a
GM TSB for F23 noise, and they specifically recommend a 2.6qt fill to mitigate. They also mention that overfilling beyond 2.6qts can lead to fluid leaks out of the breather/fill port, which is interesting in that the caution isn't with regards to leaks inside the bellhousing.
I'm not sure where you're quoting from, but for what it's worth I've never come across suggestions that 2.6qts causes more problems than it solves.
If you're getting fluid leaks into the bellhousing with 2.6qts of fluid and not at 1.8qts, and your input shaft seal and throwout bearing face seal are in good condition, then that's a good data point for folks to learn from. Sorry I can't be of more help.