Taking the lid off, is by far the best mod for the 525!
From there, the gains get smaller and smaller, but a pipe did help mine out.
2009 OUTLAW 525 IRS.
41mm Carb with Transition Ring, Ceramic Coated Header, HMF Competition Series Pipe, Custom Full Intake, Polaris Bob Oil Cooler Adapter with Custom Cooler, Rath +3 A-arms, ALBA Anit-Vibe kit, 15 Tooth sprocket, Pure Polaris Nerfs, Pure Polaris Handguards, Pure Polaris Front Bumper, Fox Float Evol X Shocks (front), Fox Podium X Shocks (rear), PRM 6 pack rack, PRM Rear A-Arm Guards, Pit Posse Shifter, Daytona Digital Tach, G-Force Wheel Spacers, IRazr Radials, Thumper Racing crankcase vent, LM1 A/F Meter. 48.6 WHP, 37.53 Torque.