"I don't forward anything nor know how to."
So are you using UPnP -- let me guess you don't even know what that is.. Did you test in utorrent if your port is open? So if your ports are not open your speed is going to be crap! Your lucky your seeming 1.5mbps - must be popular swarms, because somebody that does not send back gets ###### from the swarm.. How p2p works is sending back to the swarm what you get, etc.. If all yo do is take - if your ports are not open, after a while nobody is going to send you anything.. So for it to work - you must be on big swarms with lots of seeders, since seeders don't care if you send or not..
Like I said lots of variables in p2p to what speeds are capable..
As to a N router - I picked up the TPLINK TL-WDR3600 for like $42.. I put dd-wrt on it, and have been very happy with it.. But I don't use it as my router -- just an accesspoint.. So I can not see how well it will work as actual router running native firmware. I doubt you have the desire or skill set to put 3rd party firmware on what your buying. But it gets decent reviews and is cheap
Like I said I got if for $42
Again - do your clients even support N? How old is your imac? Why in the world would you be wireless if its in the same room as your router?