Probably the 0.047"
With a 128 bar reg pressure, the 0.047 was around 650fps on MIN and 870-880fps on MAX with the 0.20/18.9gr NSA slugs. Major difference being the .20 barrel has a bigger transfer port and larger cross section projectile for the air to push on. Just an example of the change in fps available from the wheel when the gun is adjusted with that in mind.
For your 23" 0.177 barrel, I would think a reg pressure around 120-130 bar and a 0.047" wire diameter hammer spring will give you the largest variance in fps when spinning the power wheel. That goes for pellets/slugs in the 10-13 grain range. And the OEM transfer port size is probably best for this weight.
If you're thinking of shooting slugs/pellets in the 16-20 grain range you might need the 0.051" wire hammer spring. It takes a lot of oomph to push the heavy .177 projectiles. If your primary use is gonna be heavy .177, you might talk to your machinist buddy about making that tp oval for better air flow.
My Ghosting has been with the shorter (Carbine) .177 barrel, a 18" .20 barrel, and a 23" .22 barrel. For long barreled .177 Ghosts, probably nobody out there that could advise you better than
@SorenDrost. Hopefully he'll see this and chime in. The best information comes from guys with first-hand experience.