Your ISP speeds are well with in the realm of this routers handling. However the ISP speed package you have may not be enough for some gaming and additional streaming. Some streaming requires a minimum level for download now days. Gaming and pings maybe impacted by this as well. Do you have access to higher ISP speeds?
FW releases have not been steady as we would expect them to be. There has been some issues on the Qualcomm side of SB that have impacted FW releases. D-Link is aware of these issues and is working to maintain and fix the issues. The last Beta is pretty good. There are still one or two problems we are seeing however they do not effect how the SB and the router work overall. I'm keeping tabs with D-Link as best as I can to help them with any testing and information on the router and to help users with any problems seen. There is this error being seen on some routers when trying to save a configuration. Not every one seems to exhibit this error. And on my router, ABE fails to complete it's speed test run so I have disabled it and just have SB enabled and thats working well. Not everyone is seeing this problem. So it maybe HW specific.
I'm currently running the 5500 and have my 360 connected to a DIR-868L in bridge mode thats connected to the 5500 on AC. Gaming has been great for me. Little or no lag seen while gaming last nite.
Yes the 5500 has seen some problems since release. The 4500 has some as well, however being a new QoS engine with Qualcomm and everything that goes in to that and the new UI, theres been a learning curve for sure. If you have a store that you can buy one that has a refund policy, I highly recommend getting one from a place with a refund policy. Test the router out and see how it does and if it meets your liking.
Let us know if you have any more questions and what you decide to get.