I made this upgrade list for Spyders, but since Piranhas and Spyders are nearly identical--most of these upgrades will work between the 2 guns.
1. barrel: Dye, Lapco, or CP-just make sure it fits the paint you use most often
2. regulator: CO2- Mac Dev Gladiator, Palmers Stabe, WGP Regulators HPA- Mac Dev Gladiator, AA Messiah, AA Vigilante, AKA Sidewinder
3. Revolution- 12volt
4. Trigger job or eLCD/Hyperframes
5. High Pressure Air: Compressed Air/Nitrogen tank
6. Make your gun comfy: Grips(DYE Stickies are my favs) + Drop Forward(there are plenty, get one that suits you best-these make your gun more balanced)
7. Polecat Straight Feed(if you want)
-The next few upgrades are if you go low pressure:
8.High flowing vertical adapter: the stock one is too restrictive so either drill it out, or buy another one-AKA being the best
9. High flow valve: (any turbo valve) AKA being the best(although it won't always work in Spyders with the slimmer strikers), I use a Venom Turbo...
10. Bolt:stocky works fine, even better if you take the "venturi" insert out-AKA Lightning if you insist going super low
11. Spring Kit: maddman or 32 degrees, for fine tuning a low pressure system
12.Low Pressure Chamber: they all really do the same, but the stock one is just ugly.
-There are also things you can do to your marker without buying anything: sand down your bolt face(helps reduce ballbreakage), take out any filters that kingman has thrown into the lines(they are restrictive), if you have a shutter take the damn expansion chamber off. There are also things you can do to help your gun be less restrictive, polish your internals and the inside of your reciever, drill out your vertical adapter, take the venturi out of your bolt, and modify your stock valve pin..
Also check out the other Troubleshooting posts for modifications. You can find em here www.suprbugin.homestead.com
, and Cookie's site at www.spyder-faq-center.com
You can get all this stuff at places such as www.g3pb.com www.countypaintball.com www.animalpaintball.com www.paintballgear.com