I guess my Google-fu was stronger than yours today, because I Googled for broadcom Wifi chip in macbook air mid 2011 and found an iFixit teardown that identified it as a "Broadcom BCM4322 Intensi-fi® Single-Chip 802.11n Wi-Fi Transceiver".
And then I searched for broadcom 4322 windows 8 and found this Microsoft Answers article, which may help you:
Slow WiFi with Broadcom 4322 and Windows 8 CP
Basically, that user had similar slow performance to yours, with a different preview of Windows 8, and found that the BCM4322 driver Microsoft had included with that version of Windows 8 (184.108.40.206 from 6-Jan-2012) had problems, which he was able to resolve by force-downgrading to version 220.127.116.11 from 06-Apr-2011.
It would be interesting to know which version of the BCM4322 driver Microsoft distributed with your version of Windows 8, and which version Apple distributed as part of the current Boot Camp Windows driver package.
By the way if that doesn't solve your entire issue, I should point out that you may have two issues here:
Why are you only connecting at single-stream, HT20, long guard interval (long GI) rates? Are you connecting to a single stream, 2.4GHz-only AP (on of those "N150", non-MIMO, N-in-name-only cheap crappy APs)? All Macs with N support have always had at least dual-band 2x2:2 support, with short GI in either band and HT40 in 5GHz (but Apple devices only do HT20 in 2.4GHz as a "good neighbor" policy so that things like Bluetooth still work), so you should be able to get a 300mbps signaling rate with a good AP.
Why is your throughput so slow? Even if a cheap/crappy AP explained the 65mbps signaling rate, you should still be able to see TCP throughput of at least 33 megabits/sec at that signaling rate, if you're measuring properly.
So if you need to troubleshoot this further, I suppose standard Wi-Fi troubleshooting steps apply:
- Try a different channel (use a 5GHz channel in 40MHz mode if you can).
- Make sure you have strong signal.
- Eliminate sources of radio interference in your environment.
- Make sure your AP firmware is up to date.
- Do your speed tests using a tool that uses TCP efficiently, such as IPerf, between your wireless client machine and a wired Ethernet machine plugged into the LAN port of your AP. Don't introduce uncertainty by trying to measure performance using software of unknown efficiency, or by trying to measure wireless-to-wireless, or by including your Internet connection in the speed test.
- And in your case, you should probably see if this works fine under Windows 7, since I don't think Apple has announced Boot Camp support for Windows 8 yet. Apple may be planning to update Boot Camp with Windows 8 support once they get fully Windows 8-tested drivers from their chipset vendors.