Weakest Link - more
Finally, combining AC867 and A54 clients produced the highest variation in AC throughput, making it difficult to eyeball what I'm calling as an 140 Mbps average (30%). The 802.11a's drop from 17 to 6 Mbps was the highest at 65%.
Mixed network - AC867 & A54
Pulling all this into a comparison table brings me to two conclusions. The first is that I should have found another N150 client to run the experiment with. There is apparently something limiting the iPad from reaching the maximum throughput it should be able to reach with an N150 radio. So we're not really seeing the throughput reduction we should see in the N150 client's performance during mixed operation.
Table 3: Class combinations and throughput losses
The second conclusion is that 802.11ac seems to handle "legacy", i.e. 802.11a/g, clients better than N did. Throughput loss always was lower than 50% and that is from its highest levels. The result of running the same experiment with 40 Mbps throughput caps in place is shown in the plot below. The AC867 client continues to chug right along at 40 Mbps with just a bit more variation. And even the 802.11a client's throughput reduction is lessened from 65% to around 40%.
Mixed network - AC867 & A54 - 40 Mbps cap
In both scenarios, however, it appears that the correct tradeoff was made by attempting to minimize throughput loss on the newer, faster client at the expense of the older, slower one.
A lot of work went into the development of 802.11ac to incorporate lessons learned from 802.11n. There are many features built into 802.11ac aimed at making spectrum use more efficient and reducing overhead when dealing with mixed networks. If you want to learn more about this, Matthew Gast's 802.11ac - A Survival Guide, is about the best resource around.
So what's the bottom line? I'd say that most people will be fine connecting the mix of devices they are likely to have, i.e. mostly N devices, to their new AC class router. The only case where I would advise an add-don't-replace strategy is if you have a lot of "legacy" a/b/g devices that are used for streaming or prolonged downloads.
However, keep in mind that, as shown in Will A New Router Really Improve Performance, you need AC clients to get the most out of your AC router. Only then will you get the maximum total throughput that AC is capable of delivering. See this article to get an idea of what that might be.