I am running LEDE 17.01.4 on an R7800 and recently had the experience to use DD-WRT v3.0-r33770M kongac (11/15/17) on a friend's R7000. There are quite a few more driver-level options exposed in DD-WRT, at least on the Broadcom. For example, checkboxes for QAM256, Beamforming, and Airtime Fairness to name a few. Are these simply enabled on LEDE 17.01.4? Perhaps I am missing the options in LuCI?
I read through that and other pages prior to posting but didn't see any mention of the options I asked about in the original post. What am I missing 
Broadcom chips are not as well supported by LEDE as dd-wrt. dd-wrt has better drivers for those chips due to a license agreement with Broadcom to use proprietary drivers.
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Some are controlled through debugfs. There's usually no point in touching those options though. The dd-wrt interface might expose those options but there's no guarantee they do anything (DD-WRT's quite hacky of a code base).
Meanwhile LEDE offers options like 802.11w that DD-WRT does not.
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To directly address:
- airtime fairness - WMM must be enabled for certain wifi modes...it provides "Quality of Service" on wifi...what other "fairness" are you referring to?
- QAM256 - this is used on 802.11ac devices in certain MIMO configurations, if you possessed a checkbox to disable it, you were probably lowering your bandwidth on that firmware. See: https://en.wikipedia.org/wiki/IEEE_802.11ac#Data_rates_and_speed
- beamforming - again, this is a feature of 802.11ac, if you had a checkbox to disable it, you were probably lowering the bandwidth of your AP using your previous firmware (or allowed to configure the AP to non-regulatory specifications, depending on country, of course).
UPDATE If you are refering to SSID fairness on a channel. Again, this is required in some regulatory domains (e.g. the US-FCC). There are various advanced settings that change these (in certain country settings), but they are enabled by default for regulatory purposes (where applicable in each country setting). Some of the configs you describe can place an AP into an illegal mode that causes INTERFERENCE. Be mindful of the configs. Don't make the mistake of the other firmware manufacturer, the LEDE developers didn't.
If you set your AP to 40 MHz for example, it will remain at 20 MHz unless airtime fairness determines the channels is clear, it can then use various data rates, up to QAM256. That mode would use MIMO, and beamforming would work.
Airtime fairness is completely different. It has to do with the amount of time an AP allocates for a given client. On ath9k (at least), it's enabled by default
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@lleachii - Very nice explanation. I am glad to hear that these options are enabled by default on LEDE... both simple and elegant. I just found myself thinking they weren't an option since I didn't see buttons to enable them.
...it's too bad LEDE doesn't have the same relationship with Broadcom as DD-WRT does. There are many Broadcom-based devices (R7000 for example) that need to run DD-WRT in order to enjoy a modern kernel.