Re gear it, you will lose a bit of top speed but it will pull your arms off from 0-70. As for modifications, proper setup is your best bang for your buck. rejet for the coldest conditions you will ride in and then install a tempaflow to regulate the jetting. Its a device that monitors air temp and adjusts the jetting accordingly, basically a "set it and forget it" regulator. Proper clutching is also a huge improvement over stock. These two things alone with the increased traction of a 136 and regearing will make your sled feel like it has double the power, without adding any power adders such as a pipe.
My 700 was literally night and day, it will beat most stock sleds up till i reach top speed, then they will go by. Stock apex's, f7's, 800's, ect are no problem, im not joking. This should be your starting point, and if you feel the need for more power, depending on your budget, you could do a 660 kit, pipe, or both.
2003 pro-x 700;
TracksUSA 144" tip-up rail extensions
1.25 camoplast predator /120 1.40' extreme max SS studs
VanAmburg tunnel extension
SLP Powder-Pro skis, Powder-Coated suspension
Holtzman tempaflow-420 main jets
10-64g weights w/ round almond spring
24/42 gearing, gutted airbox, PERC, torque arm