Sorry to hear about your issue with hearing on the phone using the Comply foam tips. I’ve ordered me a set from Amazon after hearing your great testimonial here. They arrive tomorrow. I’ll try them out and report back as well and compare notes.
Regarding the 15-20% volume loss, I’m guessing that it may be because the memory foam is more porous than the silicon power dome, and that’s why some of the sound may escape. That’s probably what makes it more comfortable in the first place, because the air pockets inside the foam allows soft but conformity to whatever shape your ear canal is. But with air pockets comes porosity and therefore lets not just humidity but also sound escape. If it’s simply a volume loss across the board, you may want to try to simply increase your volume buttons a notch or two to see if that would make you happy again. If yes, it’s a very simple matter of asking the audi to increase the overall default volume next time you visit him to your liking.
There are settings in the OPN for the different types of domes to factor in the sound escaping (like in open domes or closed domes with single or double vent holes, etc). My audi fitted me with closed dome with double vent holes originally but it has some feedback issue so I switched to a closed dome with only a single vent and the audi didn’t bother changing the settings back to the original closed dome with single vent hole. But I can’t really tell any difference between those 2 settings anyway. I know there’s no built-in setting for foam tips but maybe your audi can try to use the setting for closed dome with vents and see if this helps boost up the volume and compensate for sound loss. I’m guessing that you either change the dome setting or adjust the default over all volume. But you can try manually adjust the volume yourself first to see if you’re happy with that fix before making it permanent through a program change.
Comply also makes another type of foam tip called “Isolation” tip (instead of “comfort” tips). This type is designed to block more noise out compared to the comfort tip. It has a little bit longer, bullet shape tapered design to create more contact area with the ear canal to block out noise compared to the comfort tip. You can see that the shape of the comfort tip promotes less contact area (only in the middle) compared to the isolation tip. If it helps block more sound from coming in, I assume it would also help block more sound from coming out as well. So this may be another option to try out, beside the volume adjustment or dome setting change. However, I saw a Youtube comparison between comfort and isolation tips and the user said that the isolation tip may have a higher tendency to get squished at the tip if your ear canal is not long enough to accommodate it. If squished at the tip then it can block the sound coming from the receiver tip. But if you feel that your ear canal is long enough and this wouldn’t be an issue then it may be worth a try.
Then there’s also a “sport” foam tip which is designed for wearers who exercise and/or sweat more inside their ear canals.