Custom combs worth it?

My personal opinion is that custom combs do little or nothing for most harps. A harp with a swollen, warped, or cracked wood comb is better with a new comb, but if the comb is in good condition I don't see any reason to replace it.
One exception might be Brendan Powers combs for chromatic. He makes them with small ramps in the reed chambers and these help according to some. I think custom combs came about because of problems with wood combs. Manufacturers have addressed the issue and modern combs perform well and last a long time.
15 or 20 years ago there was a fad in the harmonica world that had people making gaskets for diatonics using micropore tape. This was to make them more airtight. Nobody does this anymore. The fad came and went. I tried it because of all the claims being made but I found no improvement over the stock harp. Modern harps have good combs and are airtight. I have many Lee Oskar harps and they all bend very well.

I don't really know enough about custom combs to give a learned opinion. I have some harps with custom combs that are wonderful harps.
Pete is very knowledgeable on this subject since he customizes harmonicas. I have a couple of his custom harmonicas and they star playing before they hit your lips. Maybe he'll see this post and respond.

To the best of my knowledge no one is offering custom combs for Seydel Orchestra, it wouldn't make it louder anyway.
If you want louder, doubling the reed plates would be a good start.
If your harmonica is not responsive because it is leaky, it won't be as loud as it normally would be. Check the reed gaps, too wide a gap needs more air to facilitate the reed. This often the culprit for new harmonicas, beginners tend to blow too hard. A rule for default gapping is this: Make the gap the same width as the tip of the reed.
Harmonicas because of their nature will only get so loud. Beyond that, more breath force would increase the presence (fuller) but not the volume. Anyone who has played with a loud guitar player (aren't they all?) can attest to the fact that more volume is just not accessible without amplification or a projection device. The idea behind a custom comb is very sound, flat mating surfaces don't allow as much air to escape into adjacent holes. So if your Sydel has a wooden comb it would be possible the flat sand it, and the bottom reed plate to make for a better mating surface.

Matt, try dropping your jaw when bending low notes. Those bends shouldn't require more effort.