The first thing the OP should check is the regulator setting. That requires no disassembly of the gun but does require shooting over a chronograph starting around 150 bar. Velocity will fall noticably at some point and that will tell you the regulator setting. Look at the pressure gauge on the end of the air cylinder when the velocity is lower. If the regulator is low, like 120 or even 130 bar, you can definitely get more velocity by increasing it. If it is 140 or more, there is something else wrong.
I have a P35X I was gifted by another member when he had a similar issue with it. It's problem is SPA drilled the passages through the gun as if it was a 22 when it is a 25 caliber. This is not your issue but illustrates that the flow area through the gun is a big deal. I may yet fix it but haven't so far because the valve doesn't want to come out. But if your regulator is not really low, then I would start disassembly and look first at the barrel port. SPA normally drills them at 75% of the barrel diameter. So about .133 inches. My P35-25 was drilled at 75% but had a burr in the barrel port. Removing it gave me measurably more fps. I have increase the area of the barrel port in my P35-22, P35-25 and P35X barrel to 85% of bore diameter. That will give you a little more. I also have elongated the hole so it is longer than 85% in the direction of the pellet travel. You are limited by O-ring grooves but I got reasonably close. You won't get more than maybe 20 fps with this sort of modification but it can help a little. If the hole is poorly drilled you may get more.
There could be an issue with the passage through the block between the valve and the barrel but I know of no way to fix that right now. I am going to try heating up the stripped down block in my oven and see if the valve will come out when it's hot. The block is aluminum and the valve is SS, I think, so differential expansion may help. If I can get the valve out I will drill out the P35X block I have to 85% of the .25 bore area. I messed up the valve trying to get it out, however, and it leaks so I will have to fix that too for the gun to work.
The valve in later bullsharks (or some bullsharks) and my P35X is brass, not plastic like my P35s. The hammer spring of the P35X is made of 1.3mm wire, not 1.2mm wire like my P35s have. I haven't tried setting the regulator of brass valve guns to higher regulator pressure but it seems like they should be able to handle more. My P35s run out of hammer spring at something less than 150 bar. My guess is the P35X can handle more, maybe 160bar, maybe more.
With the long barrel you should not need a super high regulator, however. You have a 531mm barrel and a big plenum so for you to only get P35-177 level power is not right.
If you need to change the regulator I've explained that several times and you should be able to find it in my previous posts. It is not hard although anything you do for the first time will take a little longer. Maybe 30 minutes the first time and 15 minutes when you have done it a time or two. There are no markings to tell you how much to turn the adjuster screw. If you are really low I'd turn it at least 1/2 turn in (clockwise).
