I've owned five Beeman HW77s, two bought brand-new and three bought used. The brand-new "Supertuned" HW77 I bought from Beeman not only shot way below his published velocities, but slower than all four others. Yes, I understand tuning is not always done for higher velocities. Nevertheless I would answer your question with, " somewhere between not very good, and snake-oil good".
Based on that I'd say something was wrong with the rifle. Every Beeman airgun I've owned lived up to the rated velocity. And that without a super tune.
My experience is right in there with AirnGasMan. I never had a single Beeman labled rifle or pistol in the 80s and 90s that met his marketing specs. NONE... EVERY RWS I bought met or exceeded theirs. He disparaged pumpers and other brands such as RWS frequently . I always found those to be dependable, accurate , and well made, as well as meeting their advertised specs. I bought a LOT from Beeman(long running airgun addiction). That includes the Supertuned and Laser versions. I spent a LOT on tuning kits, etc to try to achieve his published specs.
Now that said, they were mostly wonderful rifles and pistols and I was just fixated on velocity... just like a lot of newbies are now. Beeman was just a great marketer.
I know that David Slade worked for him in the 90s. I'm not sure I had one that he tuned. Since knowing David and having other work from him, I would expect that if you happened to get one he worked on, you would be happy.
Interesting that I still have a couple of his tuning kits. Still have a few springers... R1, FWB124 and 300, BSA Superstar , and some cool Mendoza's waiting to be restored.
Fun stuff !
Bob