Can mixing RAM memory size (on its own) reduce dual channel performance?
You will not be able to achieve dual channel performance with modules that are not identical.
What you get is called by intel FLEX mode.
That means, if the MB is good enough, you still get faster-than-single channel operation, but not entirely dual-channel operational speed. Motherboards that support FLEX mode will list it as "dual channel" regardless, so that's not an accurate indicator.
You can see FLEX here.
Basically, the common dual channel area will work in dual channel, while the difference of the larger module will be slower, will work on single channel.
in your example, yes, the 24GB(8+16) will behave very close like 16GB(8+8) for the first 16GB of memory used.
For things like HANA you should definitely use identical modules.
So Far, I used 16 + 8 Gb RAM on Dell Precision 5510 and Desktop with Ryzen 2400G, They both run at dual channel with mixed size RAM. It is faster than single module and in real usage no difference with 8+8 or 16+16. Only in benchmark the number will be 150% of single channel with one module. The reason is simple, it runs dual channel up to 16 Gb and single channel on the rest of 8 Gb. Sisoftsandra values about 18 Gbps memory bandwith. With dual channel it reach 25 Gbps, single module 13 Gbps. So just upgrade to biggest single module you can get and keep the old RAM. 24 Gb total RAM is better than 16 Gb.