What will it buy me?

I don't know, but where did you get the custom romex?

John, I don't know if those re-positioning of breaker steps will really make any improvements because all the breakers are parallelled onto the buss bars anyway. Both on hot and neutral. So there will be common connection between them no matter what their relative positions. I suppose it is worth a try to see if anything is gained.
The dedicated circuits seem to have been reported by other members to have good effect, so that is more likely to have the effects you want.
Re-wiring your amp and providing a 230VAC 20amp line will have the effect of using larger diameter wires, since the current will be halved at the 230VAC. If you had any current limiting of the amp from the wiring system, it should be gone with that mod.
Can't comment on the PS or FIM outlets. Never used them.
If you are going to go through all that wiring work, you might consider just making an entire separate sub-panel specifically for your audio gear. You might even use an isolation transformer between the main panel and your audio sub-panel. You could even place chokes and caps for filtering directly across each breaker for filter-isolating between individual circuits. Running a separate ground cable from the audio sub panel to a separate 8' copper ground rod sunk into the ground outside could help too. This would isolate your audio neutral and ground connections from the rest of the house. Using a metal sub panel and providing a good ground connection to it could act like an RF shield for your audio circuits breaker area.
Running all the Romex at least one foot away from all other Romex in the house, will minimize or eliminate any inductance hum or noise radiating from these other circuits. I have read somewhere that one twist per meter of the Romex will provide some RF rejection also. Grounded metal rough-in boxes instead of plastic can also provide some shielding effect at the outlet areas. You might even try grounded metal cover plates. Just a few suggestions, and I wish you luck and success in your efforts.

Thanks for the insight, Twl. Jsbail, I picked up the romex from a friend who got a spool of it custom from some wiring company.

10 gauge 99.95% OFC is not that uncommon though. Check out JPSLabs and PartsExpress.com for like wire. But as you probably know, good romex is still an extremely inexpensive upgrade.

Stehno, I think you are ready for some ESL or Planar speakers?! Aerial are beautiful sounding, however it is time to evolve from the wood boxes and to let music free. Think, Soundlab.
Rgds!