The WA module is certainly a big help for the portal. In some of my tests I've seen improvements that speed up the portal by twice as fast for certain operations. Perhaps the cost/value calculation will show that WA will pay off for your company.
Short of that though, I'm wondering if you have compression turned up for javascript, it's not clear from your message.
In order to enable compression, you would use an HTTP profile and add at least javascript mime type to the list, if not other mime-types, and then associate this profile with the virtual server. I've run all of my tests with IE6 and the compression improvements with javascript are definitely helpful.
Also, have you tried taking advantage of RAM Cache in the HTTP profile as well? Some gains can be achieved for javascript as well by reducing one more hop and serving the files from the BIG-IP's memory instead of the SAP server's disk.
I'm also wondering how your server loads are looking like? Is there more the BIG-IP can do to offload server load for you? Is one-connect turned on? Are you off-loading any SSL to the BIG-IP. Anything that can help the SAP server provide more cycles could help.