1. Depending on the source, streamed 4K is usually about 15-16Mbps maximum. Back when I had Netflix, I watched a test video that showed its bitrate and resolution in real time. It would switch to 2160 (4K) at about 9 Mbps and max out at 16. Someone recently reported that they now max out at 15. I don't now for sure ho high Amazon goes at 4K, but most likely about the same. The only time you'll likely see bitrates over 20 Mbps is by streaming media from your own server. My ripped UHD movies can exceed 100 Mbps, and I do encounter buffering with my Roku players. But it's more my wireless access point isn't doing its job well. Since my Shield has a Gigabit Ethernet connection, it naturally plays them fine. Don't forget that 802.11N is capable of close to 300 Mbps on 2.4GHz, so I wouldn't be too concerned about it. The bigger issue is how fast your wireless connection is to the rest of your network. If it's only Fast Ethernet (100 Mbps), then it's possible you could max out your wireless with multiple devices using it all at the same time. But if it's Gigabit, and your Internet speed is fast enough, there shouldn't be an issue.
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2. The Express and Premiere+ are both reported to have the same CPU. I don't know if they have the same amount of memory, but most likely the Premiere+ has more. But yes the Express has an IR remote and can have issues with the beam being blocked. The The Premiere also has an IR remote, but the Premiere+ has a WiFI Direct remote. I don't notice any lag using my Premiere (not a + model, so almost the same as your Express).
Dan
Roku Community Streaming Expert
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