I've actually found a tin of Meisterkugels --- I'll give them a whirl when I get five, Drew.
From memory, the factory Spring was 33coils (32 active) of 3.1 wire, the HW95 Spring I've used is 28 coils (27 active) of 3mm wire with a free length of 230mm.
Inside the piston I have 5.25mm of preload in the form of a Delrin top hat, bronze slip ring and the thickness of the steel piston liner.
On the spring guide I have a stack of six Dowty washers giving a further 12mm of preload, so overall I have 12mm of removable preload on the spring.
If the gun was easier to strip I'd remove the Dowty washers and see what power the gun ran at without them as it could be over sprung, but as it's running well with them in I'm leaving the gun alone for now.
All the best Mick