I've got a few bits that might help:
First off, the best thing to do is to upgrade your power source (the magazines) here's a few bits:
Magazine shims, these'll help with gas efficiency.
http://www.leesprecision.com/products/laser-cut-mag-shim-pack-of-4-tokyo-marui-pistols
A better quality gas route seal will also help with efficiency, and is easy to install.
http://eagle6.co.uk/shop/catalog/product/view/id/573/s/laylax-nineball-magazine-gas-route-seal-aero-packing-2-pcs-for-hi-capa-p226-series/
This tool is essential when replacing o-rings on the gas inlets/outlets incase the magazine happens to leak after a while.
http://www.milspecsolutions.co.uk/we-gas-magazine-valve-key-tool.html
And we do love o-rings. These are probably better quality than the WE o-rings.
http://www.milspecsolutions.co.uk/gas-blowback-parts/Tokyo-Marui-Magazine-Parts.html/Lees-Precision-Tokyo-Marui-Pistol-Magazine-Repair-Kit.html
Now, parts for the pistol:
The WE bucking is ok, but there are better one's out there, like this one.
http://eagle6.co.uk/shop/catalog/product/view/id/570/s/laylax-nineball-wide-use-air-seal-chamber-packing-for-vsr-l96-gbb-series/
Replace the barrel with a tightbore, you'll get a bit of a fps boost and some slightly better range / accuracy, remember to clean it before you put it in your gun.
http://eagle6.co.uk/shop/catalog/product/view/id/1479/s/laylax-nineball-6-00mm-power-barrel-97mm-for-g17-g18c-p226-gbb-series/
A stronger recoil and hammer spring will make the gun a lot snappier, if you're not sure about taking your gun apart then avoid this.
http://www.zerooneairsoft.com/product_info.php?products_id=3660
Hope that helps.