Hi Murat,
The speargun that I use most often is 100cm with a 150 cm shaft of 6,3mm diameter and 18mm slings. It's a pretty fast and accurate setup for me.
As I said, the longer shaft improved my aiming a lot and specially confidence in shooting.
When I do aspecttos I generally have my speargun along the side of my arm or haft stretched laid on the bottom and I only stretch my arm and gun when fish make a pass.
Sometimes this has to be a pretty quick movement and you don’t have much time to aim by aligning the handle and the muzzle. Just point and shot.
I guess if you use light shafts (6,0, 6,3 or even 6,5mm) you wont notice much difference in the speargun weight, and get used to it quickly. You should try it and see if you like it and helps.
Here is a picture of one of my last hunting sessions of last year in Azores. Notice the small barracudas. Try hitting that at almost the end of the speargun range!! Those are really small and I usually let be, but there where some cats starving near my entry point.
Anyway, they are really small targets. The other fish was shot inside a huge lava tube almost in the dark (no flashlight at the time) therefore the bad placed shot.
cheers,