That's funny, someone asked me about the same thing in private some days ago. I had a look and it turns out that the settings manager has a 999.99 KB/s limit, however the mod itself does not. So if you open the serenity_settings.cfg in Notepad and edit the corresponding lines for the profile(s) you want to edit (fakeupload_min and fakeupload_max), you can change them to whatever you want. I tested 12 MB/s and it worked fine.
A quick attempt at patching the limitation in the executable failed, but it's not like I really tried; editing the configuration file by hand works fine, and in any case, for most people who need a fake upload feature in the first place 999 KB/s is a distant dream.