Often, in fact pretty much ALWAYS, the upper mid "fizz" is essential to maintaining any kind of brightness in a high quality guitar tone.
While it seemed acceptable to have a guitar tone very small on core mids, not a hell of a lot of lower mids and a fair chunk of mid bass back in the 80s, along with a healthy does of horrible treble fizz, these days it's not really the sound anymore, unless one likes typical shitty 80s thrash metal guitar tones.
If you can manage to beat the nasty fizz, while retaining the fizz that makes a guitar retain brightness, then you're on the money basically.
To me, it makes more sense to attack fizz in the highest registers where you are hearing fizz most.
While a LPF will tame the unnecessary "air", often, as you know, that's not really enough anyway.
Start by taming 8KHz, 7.5KHz, whatever offending frequencies you can find in the treble range. Don't cut too much obviously, or at least a very large Q so you're not starting to cut into the upper mids otherwise you get muffled
Once you have that done, only then should you really start fucking around with the upper mids.
Upper mids is a pain in the ass. This is really the core component of brightness IMO, so be careful and constantly reference your tone against benchmark productions that have super controlled treble and awesome high mids (examples are all 3 Paramore records, Linkin Park's Hybrid Theory, a fair amount of Sneap's mixes and various Nickelback and all that stuff etc)
