To increase power a PDI 6.01 barrel should give you a boost of 15-25 FPS right there. A Nineball 6.00 power barrel might give you a few more FPS, although the difference will likely be slim. Assuming you are using 0.20g BBs, 260 FPS is kind of low for a P226, it might have a leak somewhere. Since you already replaced the piston head, it would be wise to replace the hop-up rubber too. If you want maximum power the Nineball purple rubber is the best, although if the hop-up rubber is the problem, any good rubber will give you a large FPS boost.
Also, it is possible that there is a problem with the mag release, that causes the mag to sit a bit low, causing a leak. If you press the mag up in the mag well with your hand while shooting, does the FPS increase? If not, maybe the nozzle is cracked, causing a leak. To test this, take the loading nozzle out and gently bend it and put pressure on it in different places. If there is a crack, you should see it that way.
One last thing that could help is using a stronger hammer spring. Older/weaker/worn hammer springs won't release as much gas when they hit the valve. Using a stronger hammer spring will lower gas efficiency, but it does improve FPS a bit (or a lot if the one you are using is too weak).
With these upgrades, you shouldn't have to worry about the plastic slide, it can handle it (my 1911 and hi-capas can handle it just fine, and I have never heard that the P226 is any different).
Keep quiet. Sound travels faster than BBs.
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