All your info and use should be found here, first post, If there is any other info you need please ask. Why do you need to telnet into it? (It is very easy though)
http://www.snbforums.com/threads/dd-wrt-for-rt-ac87r.22421/
What are your settings for the 5 GHZ
My devices (IOS specially, all on IOS 9 beta 2 now, but no problem with Asus) don't seem to like it
It works at first reboot then they do not connect to it. Also my Samsung TV chugs to find connection but it does use the 5.
Beam forming on the 2.4 is great.
External hard drives worked the same for me as Asus firmware when I used them, Please consider a NAS, so much better in every way, I have a QNAP and it is the best device I purchased in a long time.
No hardware accretion that I see (I don't understand what is under the hood though), I only have a 30meg. No loss there for me. I think it tops at ~350 (maybe 450) from ISP's.
No 2.4 GHz light still, maybe someday.
There is a very easy setting in the GUI for overclocking, though I don't know what the need is.
From what I have read on dd-wrt, people seem to use it for its stability over stock firmware (of course this is debatable).
I was nervous at first also, that is why I created that thread I referenced above, it is really easy to use, its just stuff is in different places, or not their like trend micro, and vice versa, dd-wrt has stuff better or not in Asus firmware, like wifi maps, and much better wifi device signal meters.
If you feel adventurous go for it, if not stick with asus or merlin, I think it is no worse than flashing stock firmware, I have been back and forth many times, I have never bricked my router, nor do I see it worse than flashing stock and bricking it. It's always possible, with any firmware.