My personal experience with building VoIP solutions for retail environments is that if WiFi is perfectly fine for physically small areas.
Your first step is to do a WiFi survey; are there any areas where you don't get "five bars" of WiFi signal strength? If so, those will be "problem spots" for call quality.
Second step is ask yourself "what codec am I standardizing on?". Review the calculator here at AsteriskGuru to get an idea. 5 calls in full G.722-HD are going to use ~796Kbps ... that's 1/4 of the transfer rate you post between just two network points.
The Big Gotcha, almost invariably ignored, is the phones themselves. Some VoIP WiFi phones are just bad -- usually poor ergonomics and no support for HD audio, with low-quality WiFi electronics. Be sure you get good phones, and there should be no reason that a small-footprint VoIP WiFi environment won't work more than well enough for an SMB.
A larger space -- such as a warehouse or shop floor -- then the WiFi environment trumps everything. You may have to invest in repeaters or signal strength boosters as part of your VoIP deployment plan. Be sure you talk to the folks who will have the phones and be sure that your WiFi Survey does things like stands behind stationary machinery or shelving stacks to ensure you don't have dead zones. This is particularly important for 911 considerations.