I have owned a 7363, R500, and now an R700. I upgraded to each from the former in pursuit of better throughput. I am now running firmware 184.108.40.206.14 on my R700 and experiencing throughput during windows file transfer that I consider to be pretty poor. I can only get around 18 MB/s in local file transfers involving a NAS which is very fast and a Dell XPS15 9550 laptop with a 3x3 card and a fast SSD. This is with sitting 10 feet LOS from the AP or in testing putting the laptop right next to the AP.
I feel like my 7363 was about this fast to a 2x2 n laptop. My R500 was a little faster to a 2x2 ac laptop, but not by much. I feel like a 3x3 AP like the R700 going to a top of the line 3x3 laptop should be much faster. I don't have any other spectrum hogs on my network and I am not in an apartment with lots of interference around.
What should I be expecting in this scenario?
Also, of little consequence to this discussion but just to note, I find the firmware situation with this AP to be a bit frustrating. I'm running 220.127.116.11.14 for stability since I had issues with the other firmware trains on my R500. But this firmware seems to have many important features disabled. Later 9 prefix firmwares seem to be better, but they are only available with premium support? I find it pretty weak that I can't get a stable firmware that has all the important features enabled. I see references to a promising 104 prefix firmware for the R700 on other AP's download pages, but it is not accessible anywhere. No bueno. I understand that I don't have premium support, but can't I just get a darn recent and fully featured firmware image?
To answer your question directly, 144mbit application throughput is a bit on the low side. I usually see around 300-400mbit from a 3x3:3 client to a R600 or R710. Are you sure you're on 5GHz during your testing? What you're describing seems like near the ideal limit for 2.4GHz.
To be perfectly honest, the upgrade from 2x2 MIMO to 3x3 MIMO often doesn't translate to as much of a real world benefit as you might think. Even "3 stream" laptops often thoughtlessly place 3 antennas near each other for the sake of checking off a checkbox, not adding meaningful spatial diversity.
Also, quite honestly, the analog side of the R500, R600, and R710 series (from the teardowns, the antennas are very similar in this line) is noticeably better than the R700 series (which is essentially a 7982's antennas on an AC capable radio chipset). I never had a situation where my R600 was beat by a R710, and performance even between R500 and R600 is very similar for the wide majority of clients I have.
It had been a while since I had really tested to probe this problem, so I tested on a few other laptops after making this post and found that even one of my other laptops with a cheesy Linksys USB 3x3 N stick can get better throughput than the XPS 15 9550 that I was referring to above. It was up in the 25 MB/s range. So I guess unless the AP has a problem that is isolated to AC (unlikely) that it's just a problem with the XPS 15 9550. The XPS has a Dell-branded Broadcom wifi card and the general feeling on the net is that it's pretty poor. I'm considering if a 2x2 Intel would be better since, as you say, the third stream often doesn't do much due to poor spatial diversity.
I might give 18.104.22.168.432 a shot, but I had problems with a 100 prefix firmware on my R500, which led me to believe that a maintenance release on an older train would be preferred to a newer (buggier) train with no maintenance release. I put in a ticket to see if the 104 prefix can get posted for the R700 since it already is documented on the R700 document page. Initial response from support sounded good but we'll see.
Great! Let us know how it works out. FWIW I had a ton of AC performance issues on earlier 9.8 releases though I haven't tried 9.8.3 which was supposed to address a lot of that. I'm running 100.1 on my R700 in my office and it delivers a solid 435mbit down and up to a 2 stream smartphone on a gigabit connection.