Ok so I couldn't really find the Prometheus 6.03 455mm, they are also double the price, so I went with the Vanaras against better judgment.
I was pleasantly surprised!
Here is the "spreadsheet"
As you can see quite a gain in FPS and it was only a longer barrel and the silencer added. I could test with both stock ICS 363mm, Vanaras 363mm and Vanaras 455mm which I ended up taking.
The accuracy increased a lot and I'm shooting pretty consistent. Distance is also quite a bit further.
The Vanaras precision barrel is a nickel plated Brass barrel. It looks really smooth inside and it's heavy. So I think it's not too bad.
Pick of the gun.
I kept the Cylinder the same even though it is only rated at 430mm barrel length. I will do a upgrade to the 4/5 one later.
For the price I paid I couldn't have asked for more.