David Luke of RevSound may well be the very best person out there for what you're looking for.
And, I'm not sure that significant extension north of 5 kHz is necessarily desirable if you're going to use heavy distortion. Distortion = a LOT of higher-order harmonic content, and I'm under the impression that the natural top-end rolloff of guitar speakers helps to keep all of those exaggerated upper harmonics from sounding too bright and harsh.
I think my experiments with running distorted guitar through my Birdhaus 48 bear this out. I use the parametric EQ, set for the 2-band shelving option, in my preamp on the patches I've tweaked for guitar, and what approximates a "high-gain dirty amp and guitar speaker sound" to my ear involves gentle rolloff below 250 Hz (or slightly steeper rolloff below 100 Hz) and--this is a crucial adjustment--gentle top-end rolloff starting at either 2.4 kHz or 3.6 kHz. I'm talking maybe -2 or -4 dB at those shelving frequencies.
Adjusting at the 4.8 kHz shelving point requires pretty big cuts to influence the crucial upper-mid distorted guitar frequencies, and to my ear sounds artificial and hyped.
Keep in mind that I'm going for sounds in the 70s/80s Marshall universe when I reach for a high gain sound, and I shy away from what I hear as spitty, woofy "modern-rock" Mesa dual rectifier-esque sounds. Those might respond better to more extreme low and high frequency content to those who like that kind of thing.
But in any case, I think you are not going to need extention much past 4 kHz to get some ripping crunch, and you might find yourself cutting quite a bit in that area anyway.
Also keep in mind that my guitar is a strat-type with a single bridge humbucker, fixed bridge and no shortage of bright, and it's a little shy on the lows...if you play a darker guitar or one with less screaming upper harmonics you might be reaching for more shaping of the highs than me. I play pretty hard and hamfisted, and find myself turning lows down on almost anything, and turning up the mids and the bright for texture...backing off just enough that it doesn't make my teeth hurt. I seem to have no shortage of ability to force unwanted lows into and out of amps...