Yes, the fastest 802.11ac equipment on the market today allow PHY rates up to 1300mbps. With standard frame aggregation, they can just barely hit 80% efficiency, for TCP throughput of 1.04 Gbps, which is better than gigabit Ethernet's maximum TCP-over-IPv4 throughput of 941 Mbps (with standard sized 1500 Byte frames).
I've actually seen two 2013 retina MacBook Pros (which support the 1300Mbps flavor of 802.11ac) do 1.02Gbps of IPerf TCP throughput over Wi-Fi when one was acting as the AP.
When bridging from 1300Mbps AC to gigabit Ethernet through a typical consumer AP, you'll be limited to gigabit Ethernet's 941Mbps of throughput, but typically also have a little higher latency and a little less efficient frame aggregation, so you'll probably only see a little over 800Mbps of TCP throughput even in ideal conditions. But that's still pretty close to gigabit speeds.
Whether or not you can maintain the 1300Mbps PHY rate in your particular RF conditions (through the garage wall, etc.) is another matter, but 802.11ac gives you a fighting chance of having gigabit-like wireless performance.
You're not going to be able to get these speeds with an 802.11ac USB dongle, because those dongles usually only do the 867Mbps (2 spatial stream) flavor of 802.11ac, and a lot of them only use USB 2.0 for some inexplicable reason, limiting them to well under 480Mbps.
But if you put a 3-spatial-stream 802.11ac PCIe card in your desktop, like the ASUS PCE-AC66, and put a 3-spatial-stream 802.11ac router in your garage, like the ASUS RT-AC66U, and you position them well, pick a clean 80MHz-wide channel, and tune your TCP window size well, you have a fighting chance of getting 500-800 Mbps of TCP throughput between your desktop and your servers in your garage.
Your AP (router) is capable of transmitting each individual packet at 300Mbps (Megabits/sec: 1,000,000's of 1-bit bits per second)…
IF AND ONLY IF the client is also capable of the same 300Mbps flavor of Wi-Fi…
…AND the AP is configured to use 40MHz-wide channels in the given RF band…
…AND if it's using the 2.4GHz band, there are no "40MHz intolerant" devices in range…
…AND either "no security" or "WPA2" (AES-CCMP) security is enabled…
…AND WMM (QoS) is enabled…
…AND the client is close enough to have a strong enough signal AND the AP is on a clean enough channel for low enough noise for the 300Mbps signaling scheme to be successfully received.
So that gives you a list of things to check on, to make sure you're making the most of your 300Mbps AP.
But that's only the rate at which each individual packet is sent. Wi-Fi has a lot of overhead, so even with the efficiency boost that was introduced by 802.11n's support for frame aggregation, your TCP over IPv4 throughput is only going to be a maximum of 80% of the signaling rate you get. And the overhead of the SMB/CIFS protocol and the latency of the software and disk are going to drag that down even more.
So even if you were getting the 300Mbps signaling rate, your CIFS file I/O throughput might only be 60% of that, for 180Mbps / 8.4 = 21MiB/sec (MebiBytes/sec = 1,048,576's of 8-bit Bytes per second).
The 2-3MiB/sec you're getting is typical of 802.11a or 802.11g networks in fairly ideal conditions. Check through each thing on that list of ANDs above to see what's going on in your case.
When it comes to making your decision of wiring up vs. upgrading Wi-Fi, remember to factor in the cost of upgrading the Wi-Fi on the three client machines. For example, if you decide to fully modernize to 1300Mbps 802.11ac (3 spatial streams, 80MHz-wide channels), you're probably looking at $180 for the AP, $100 for something like an ASUS PCE-AC66 for each desktop, and $70 for an ASUS USB-AC56 for each laptop (and the USB dongles only do the 2-spatial-stream, 867Mbps flavor of 802.11ac).
So for going to 802.11ac Wi-Fi you're talking $300-500 for the potential of getting in the 40-90MiB/sec range for your CIFS-based file transfers.
Going to gigabit Ethernet would give you the potential of a steady 100MiB/sec (that estimate already accounts for CIFS overhead), but your fileserver might not be fast enough to keep up with that.
Note also that gigabit Ethernet requires that the switch and the PCs' NICs be gigabit-capable, in addition to your Ethernet cables being Cat5 or better. It doesn't matter if you have Cat6 cable if your equipment only knows how to send and receive 100Mbit signals over it.
For me, the decision would come down to how mobile I want the Windows 7 boxes to be. If they're desktops, or if they're laptops that never move from their desks, I'd probably take the expense of wiring properly, to have guaranteed bandwidth without all the potential range/signal/noise hassles of Wi-Fi. But if one or more of them are laptops that people actually like to move around with, I'd probably go for modern 802.11ac.
Best Answer
It depends on the Wi-Fi hardware for both the router and your device; assuming 802.11ac, you'll need to check: four streams, optional channel bonding, etc., lots of things that are generally hidden under the marketing umbrella of "802.11ac". (The RT-AC66R you linked also mentioned that devices need to be joined on both 2.4 GHz and 5 GHz bands.)
Another thing that makes a significant difference is wireless interference: are there other networks sharing the same channels? If you have 9 other wireless clients, they may be consuming timeslices + bandwidth; did you turn them off before doing bandwidth tests?