Your router is only half the equation. You must look at all your wireless cards also.
It could very well be it is one or more of your PC devices causing the problem. If you even have a single G device the router will run in mixed mode and pretty much limit you to about 54m and that is maximum it is reduced by distance and interference.
To even think about going to any AC based router is crazy. First it is not standard for another year and there are still tiny differences between the vendors. So to actually use the Asus AC router you would have replace every single nic you have with a Asus AC brand. And you can just turn off things like ipad and xbox that you cannot replace the nics. If everything does not run AC you are right back to where you are with your N router. If you have a G based device it will pretty much turn you sporty new AC router into a old G device.
The current router you have should be able to perform much better it is not the router I suspect that is causing the problem. First replace all the nic with N cards rated to at least 150m This will give you wide channels. If you can find them with 2 antenna mimo you can get 300m. This is the maximum your router can run. Now if you want the magic 600m which is a lie. You would need dual band cards that could do mimo on both 2.4 and 5. But unlike the router a PC nic card cannot run both 2.4 and 5 at the same time. The router sorta can't either but they try to claim 300+300.
If you want a asus router the N66U is pretty much the same router as the AC but it has 2 N radios in it. It claims to be 900m. But it is just 3x3 mimo on 2.4 and 5
A second thought on this....how are you measuring the speed. You must measure from a wireless PC to a wired PC not to the internet or from wireless to wireless. The speed to the internet means very little it could be the internet.
After so many years of owning routers I didn't know that about the 'mixed' signal property that it automatically limited your transfer to the signal type of the oldest NIC. One of the housemates has an early gen macbook... I'm sure its a/b which would explain our shitty speeds.
After work I'll turn off mixed on my router and set up a second router to handle the A/B/G connections... if that fixes it then you just saved me $200. If it doesn't - I'll report in.
(I've tested our internet connection and we're getting within 10% of the 50Mbps line we pay for... our wifi rarely gets over 6Mb/s - its been really frustrating)
Lets not forget that a wireless AP or router is NOT a layer 2 switch. It does not create individual collision domains. It is in essence a hub. The available bandwidth is shared between devices. While most of what ufo889 said is accurate, you mentioned you had other devices. How many? And what are they doing? Gaming or streaming? Plus don't forget that the TCP/IP stack has overhead.
However the whole idea of a dual band router means that it is possible to have some clients on A and others on G.
Also environmental factors come into play such as microwaves, metal file cabinets and wireless phones. ( not cell phones mind you )