You're seeing a problem where one likely doesn't exist.
First, your cable company probably didn't "shape" traffic. They may have, but from what you describe that doesn't seem to be the case.
Your cable company was likely limiting traffic basically the same way your DSL is limited to 5000/800. On cable networks, the cable modem is where your throughput cap resides. When you're pegging your connection, queuing occurs in the modem, which causes latency to increase substantially. 500 ms with a pegged connection isn't bad at all. If I really hammer my cable modem (15/1.5 Mb) I can get gateway ping times in excess of a second. That's normal, especially if you're uploading heavily.
DSL works basically the same way.
The goal of traffic shaping is to move the queuing up to where it can be more controlled - your firewall. Once the traffic gets to your modem, if you reach your cap, it's too late. Things queue and they go out in FIFO (first in first out) fashion. Traffic shaping basically orders that traffic so the important traffic goes before the less important traffic. It's a lot more complex than that, but this post is long enough without a dissertation on traffic shaping. :)
Back to my point - your ping times from pfsense will suffer when your link is loaded to capacity, regardless of the type of connection (not just cable and DSL, T1's do it, fiber connections do it, wireless really does it, it's just how networks work). The only way to keep ping times from pfsense low would be to shape your traffic at a lower speed than your actual connection speed. Then you'll ensure at least relatively good response times. But what's the point? All you're doing is making your graphs pretty and keeping yourself from using your full Internet connection speed.
I'm not familiar with the pfsense traffic shaper as I don't use it, so I won't offer any specific recommendations on configuration.
