Sorry, do you just not understand how slit venting works, or the relationship between the impedance of the structures around the 1-2cc residual volume?
There’s a significant number of losses that can be well fitted with appropriate domes. I like custom canal sleeves too. It’s probably better to avoid expressing an unsubstantiated opinion on a preferred fitting choice as you stand a fair chance of upsetting someone who has actually found a perfectly good fitting for them with a suggestion that they should be hearing differently or ‘better’.
That’s my empathetically derived answer, or woke answer, if you prefer.