Just looking at the possibilities and challenging myself lol

In a nutshell, that TM DIR-615 are having problems and mainly with its WLAN.

The DIR-615 they provide has a lower wifi transmit power than other routers on the market, drops wifi connections abruptly and has the life expectancy of a fruit fly judging by the reports from some users since last year. It also drops VPN connections and breaks UDP connections for some applications. For example, I've tried torrenting while playing BFBC2 on the router with stock firmware and the connection to EA servers drops every 5-10 minutes. I don't have this issue when it's running on DD-WRT or when I'm on pfsense/routeros.

And no, you will hit your maximum line speed with the DIR-615. It's just that the quality of service you experience in terms of connection routing (if you P2P) and the wifi AP signal will be terribad. However, you will probably not notice this if you only use Unifi for light browsing.

Another reason these options exist is due to the fact that TM utilizes VLAN tagging to separate the various services offered by Unifi. This isn't an unusual setup however many devices do not support PPPoE over tagged VLANs even though they might support PPPoE and 802.1q tagging individually. Business users who have signed up for Unifibiz often cannot integrate their existing Streamyx systems with Unifi because of this. This is why we have VLAN bridging (using the DIR-615 itself) or you can go out and purchase an RB250GS. They both do the exact same thing once you've configured them properly however many people like to have a backup instead of relying on TM's hardware replacement team (read acbc's story on page 96 of this thread).

With low hardware life expectancy and a system which forces you to use that hardware, we definitely need replacement options. Hope that explains it

Taking into account that your're subscribing 20Mbps package, you can confirmed to reach the max speed. Most routers nowadays have at least 10/100Mbps LAN port and Wireless G of 54Mbps (router given, DIR-615 is wireless N which peak to 300Mbps)  while the speed u subscribe is 20Mbps. No matter wireless or LAN, you can achieve full speed. However, why we always complain tm's equipments are mostly because of the router. The router given, Dir-615 is problematic and may cause disconnection issue. Besides that, the router sometimes hangs. This is also why ppl upgrade their equipment.

In short, if you lucky(equipment no problem, connection no prob, no downtime), you achieve full speed
if you're unlucky(faulty equipment, bad connection), you can't achieve full speed or... you can't even online.

Hahaha~~~

rizvanrp, the more I read, the more confused I get.

• Should I buy my own router? That TP-Link WR1043ND sounds nice. Or should I reuse the router provided by Unifi? Which router would Unifi give anyway?
  
• Which is the recommended router to use with Unifi if I wish to flash it with DD-WRT?
  
• Can I use the DD-WRT provided from their official site or do I need to use your customised firmware to use all the services? It seems that you're still working on enabling IPTV support.