Your thread prompted me to dig out my old .177 28-R and give it a try on the chrono. With old seals and no new lubrication it only gave around 150 fps with 4.5mm lead balls and around 180 fps with some wadcutter pellets. So your .22 version doesn't sound so bad. I will test my .22 28-R when I can locate some suitable ammo. I am sure I have some somewhere.
Incidentally, shooting the pistol reminded me what a pain these Haenels are to cock. I know there is a trick to it, which I have mastered, but even so I would rate the Haenel 26's and 28's among some of the least user-friendly spring pistols when it comes to cocking, despite their ingenious design and engineering quality.