AFAIK, activating the ASM module itself won't add much if any load. The load increase will occur after you've built a policy and begin applying it to your VS's, also dependent upon the traffic level of those VS's. You can keep an eye on exactly how much ASM is adding under Security -> Reporting -> Application -> CPU Utilization. We are running BIG-IP 5000's and I have ASM enabled for over 200 VS's. ASM accounts for between 25-30% of cpu utilization. Total throughput on a single unit is between 350 Mb/s to 400 Mb/s currently.