Using the capture card allows me to avoid compression artifacts and a majority of network congestion (and get 120Hz!), and connecting the mouse directly eliminates additional latency and jitter. The keyboard still suffers from it, but in a shooter (like, say, ARC Raiders) mouse input is definitely most important - I can deal with a slight keyboard delay.

I may still look into those for the keyboard input, but the only improvement I foresee really making is getting Ethernet for the MacBook (I moved my one line to the PC since the MacBook's Wi-Fi is so much better). Haven't decided between a passive or active switch yet.