I’ve seen different ranges described as speech frequencies. How high is high for your speech emphasis work? “Speech banana” diagrams generally extend up to about 5 kHz for “s” and “th” sounds. Would hearing aid amplification from 6-8 kHz, which is usually the top end of what’s provided for fitting, affect speech understanding because of harmonics?
For shortwave radio work, we have limited bandwidth and live with 2.7 kHz… That’s the biggest challenge. Some FM rigs (for repeaters and most mobile uses) make it to 5 kHz but others are only 2.5 kHz. Classic telephone and VOIP/cellular gives you 3.5 kHz. Newer CODECs used in VOIP and some cellular have extended that to 7 kHz but you never know what you’re going to get and you have no control over it. Also your mic/speaker combo may or may not support that extended range but who knows.
You can find papers on the internet with voice spectrograms and indeed sibilantsssss have energy way up there above 15 kHz. For in-person speech acuity with hearing aids, it’s always an advantage to improve your response up to the limit of the device if possible. The drawback can be the emphasis on high frequency noise however. I’m ok with the equalization extending to the 4 kHz band, then rolled off above that but it’s my personal preference.