best speed for the .177 caliber ?

I think it is. My rule of thumb is to keep the speed below 900 fps regardless of caliber. Pellets are stabilized, not only by rifling twist, but aerodynamically. That's why they are shaped like they are. Once the pellet is pushed into the transonic speed range (speed of sound is around 1100 fps), shock waves begin to affect their travel. My TM1000 shoots AA 10.3 gr pellets at 880 fps and is very accurate. My HM 1000 is set at 12 fpe and shoots AA 8.44 pellets at 800 fps. This gun is super accurate, but is affected by wind a lot more. If your gun shoots tight groups, it may tolerate the speed it's set for, so leave it alone. If you can't cover a 5 shot group at 25 yards with a dime, then maybe you need to work on the hammer spring some more. BTW, don't try to compare the accuracy of a Hatsan to a RAW. Not to belittle the Hatsans, but they are not match rifles. They are very capable shooters and can shoot reasonably good accurate groups, however.

Try JSB heavy at 935 fps or 285mps. They are usually lazers out to 50 at that speed with the right barrel

H&N Baracuda Magnums 16.36gr in a .177 might bring your fps down a bit more.

Especially as (I find, YMMV) baracudas are not super efficient in the barrel

It's seems in my hatsan gladius using crossman .177 7.9 grain hollow points likes 840 ish fps for nice accuracy.i have tried 900 ish fps and the groups really start to open up .