RT-N16 ASUSWRT 3.0.0.3.78
Now available at ASUS official site ! 
http://www.asus.com/Networks/Wireless_Rout...RTN16/#download

Accordingly to source, the DM should be more stable compare to the beta 3.0.0.3.74
Already request, no news yet, RT-N56U likely 
I'm "fighting" for the RT-N13U-B1, hope they will put the same UI & DM onto it to fully utilise the USB, also as the alternate option for those who can't afford the RT-N16 ....
May I know what's your actual transfer rate (MB/s)?

I personally don't trust those Mbps as advertised/shown, I always test it physically by copying huge file (at least few GB) from my NAS wirelessly.

For the RT-N16, so far I have no problem getting 8~8.8MB/s, from previous to the current new ASUSWRT firmware.  At this moment, the RT-N16 it's the highest I've ever tested so far on 2.4GHz spectrum, this ASUS device with Broadcom BCM47xx really powerful ....

Assuming you've clean flashed, try to check your wifi settings again, eg, keep channel at least 5 away from those closest wifi spot, Enable Burst, etc ....
I have no such issues, I setup the DM as follows:

Assign a TCP port for Torrenting, use higher port say 56789, PortForward it to the device's IP, say 192.168.1.1
[attachmentid=2596555]

BT setup, usually I prefer to set limit
[attachmentid=2596557]

no problem Torrenting, even a 15GB HD movie! (no download if poor seeding or no peers)
[attachmentid=2596558]
Again, try to clean flash again before setup:
http://klseet.com/index.php?option=com_con...id=84&Itemid=79
it should work just fine!
What are you trying to PortFoward?  And exactly how did you setup?  Pictures may help to understand better....

For those routers I've tested, including ASUS brand, I always setup some PortForward as I have couple of things running behind the router.  So far I have no issue on ASUS routers & firmwares.

You may refer the above PortForward for some idea.