You can't put a number or predict on how many bbs your gun will shoot with a metal slide because every gun behaviors different as well as the temperature conditions. I know you posted all your upgrade parts, but we still can't determine the number of bbs your gun should fire because there are too many variables involved, unless if someone has a setup VERY similar to yours. And like I indicated, its also HOW you install the parts as well.
For instance, we can both be using the same Airsoft Surgeon Air Nozzle, but what if mine has a better seal with the piston head than yours? This alone will definitely affect the gas efficiency and number of bb's that can be shot out.
Yes, you saw on your youtube that the slide is drop in, but its only a reference, its not something that is certain. What if your slide came from a different batch? What if the guy that posted the video did some modifications but didn't or forgot to mention it?
Yes, you are shooting 17 bbs not compared to 62 bbs stock. Dont forget the metal slide is heavier than the plastic one. You need to figure out why it is less efficient and where the system is falling short, or if you have no clue, give it to a gun doc to fix. We can't tell you for sure where the issue is, but we are trying to help you to take the appropriate steps to diagnose the problem because we do not have the gun in front of us. We need to see the gun and how it is assembled to get a better idea on the issue at hand.
But for your REFERENCE only, I have a shooter design slide/frame, and upgraded all internals and I can fire a bit more than a full magazine.