It depends on what you value more - the bandwidth or the CPU load. All pages are dynamic. Even the main forum page updates counters once someone posts a comment. You would have to compress very frequently, and that is a bottleneck because you have to do that on your own CPU. On the other hand, bandwidth is scalable, and once it's out of your link (that you pay for) you don't care how it gets routed to any specific client.
You realize that you can Gzip static files and they don’t need to be gzipped every time they are served, right?