Icom engineers seem to continue to design their radios to be finicky when it comes to the ALC
The Icom 7100 and my 746 and even 756 had the same problem with lower than average power output as they do not want an aggressive ALC.
An Automatic Level Control (ALC) circuit governs the signal strength going into the power amplifier of the transmitter. It keeps the amplifier input in the designed range for linear operation without causing distortion.
Depending on the ALC circuit design and how aggressive it is driven by the microphone output level can cause distortion to result.
Try these settings and it may be best to use two tones or a recorded audio track to help adjust the ALC until the output power does not increase any further, then back off a bit.
Also use a little bit of compression
1. Start with no compression
2. While observing your ALC meter adjust the MIC gain so that you are peaking between 1/4 and 1/2 of full scale. Ideally with a normal voice the sweet spot is probably a third with occasional peaks at 3. If you use compression for me level 4 works out to be the best for contests
I suggest you also follow this for lower power use and especially if driving a high power amplifier where distortion products will be more noticeable and can cause QRM
I do need to take some notes but I am finding my Heils put out more mic audio than the stock Icom mics and thus if I do not check the ALC, I find if I am pushing hard on it and it does its job by backing off the power output to avoid distortion to occur.