First, the micro-stutter issue with multi-gpu setups is greatly exaggerated. It exists by the nature of multi-gpu setups, but if you keep your fps sufficiently high (over 50 in my experience), you won't notice it.
x8/x8 pci-express 3.0 is enough bandwidth for crossfire 7870s. It's the same as x16/x16 pci-express 2.0. I still don't think there are any single-gpu cards out that can fully saturate a x16 pci-express 2.0 lane.
Monitor your CPU and GPU usage before buying new hardware. Your recording lag may not be a GPU issue. It will take a hit, but recording stresses your CPU and hard drive. You always want to record to a separate internal hard drive (may not be an issue with SSDs). Youtube will only take 30fps video, so it's fine to record at 30fps unless you plan on doing slow-mo shots, then record at 60fps (so you can do 1/2 speed shots without losing quality). Recording at a lower framerate will alleviate some performance issues.