Latency spikes on wifi - power saving measure?
Yes, there is indeed such a power saving measure implemented on the Mac. This is common across WiFi chipsets on all platforms and operating systems, and is actually part of the WiFi standard itself. 

The PSM (power saving mode) of the WiFi chipset takes many forms. This you're probably experiencing is known as the "connected-idle" mode. In this mode it introduces a small latency of less than 100 ms (typically 50-100 ms) for the first packet received after the module was put in the connected-idle mode. 

Other power saving modes exist that save more power, but for example the connected-sleep mode introduces an extra latency of typically 300 ms, which is more than you measure here.

Unfortunately there seem to be no official interface from Apple to change power saving modes manually, or to disable certain power saving modes. The only work around is simply to actually use the radio for something which will prevent it from going into the connected-sleep mode at all. That could be done with for example a ping running in the background permanently. Obviously this will consume more power, which could be a nuisance when running on battery.

However despite the above, you should be aware that this power saving mode isn't really something that the end-user is supposed to or normally notices at all. Remember that the latency only affects the very first packet received after the module is put into connected-sleep mode, and that it is only put in connected-sleep mode after a period of nonuse.

When using Steam In-Home Streaming you would have a constant stream of packets to/from your Mac. This means that the WiFi module isn't going to go into the connected-sleep mode while streaming.