I was also tempted to increase the power of one of my HW55s and went down the bigger spring, nylon piston head route (from the original leather head). I wasn't brave enough to alter the transfer port size. I didn't like the end result. For the very small increase in power I had increased the recoil and lost some of the accuracy.
Didn't Weihrauch produce another full power break barrel rifle with a barrel locking system ?
Yes, a real shame.
ATB
Ian