Your Internet connection is maxed out and the upgrade path is difficult and/or expensive so why not just make better use of what you already have?
The sweet spot for Traffic Shaping is where you have a medium speed synchronous circuit that is not expandable such as EFM or 10Mb fibre Ethernet on a 10Mb bearer. You might also be locked into a long term contract with unattractive cancellation terms.
Traffic Shaping can define maximum, minimum and guaranteed bandwidth allocation per traffic type as well as setting packet prioritisation. Common usage is to limit recreational traffic so that it doesn’t adversely impact on the business critical services. It might also be used to throttle major downloads such as Microsoft Service Packs during the working day or to allocate guaranteed bandwidth for voice services that will always require the highest priority if call quality is not to suffer.
Traffic Shaping can either be done on the Firewall or on a dedicated appliance. Most enterprise grade Firewalls will offer some form of traffic shaping and traffic prioritisation QOS. If you add a dedicated appliance it would be positioned just inboard of your existing Firewall.