The only thing about Palmer's is that they are extraordinarily expensive.
How longer are you wanting to make your barrel? It might be cheaper just to buy a whole new gun, like a PM-1 or P68SC.
Also, you used to be able to find bolt-on barrel extensions for the PGP. I have no idea how effective these were in actually increasing accuracy or efficiency, but I'm sure one of those would be cheaper than sending the gun to Palmer's for permanent modification.