Dont know about Remus at all - but they seem to be popular so are probably ok. Roamer has just had a K-Tek fitted, saw it this morning for the first time. Looks very well made, twin 3" round outlets and a nice fit. And if youre looking for LOUD then its def the one for you!
The Magnex has slightly larger pipes, and its not as loud as the K-Tek (still loud enough for most people I suspect). Like the K-Tek, the Magnex seems a quality item - great fit, no rattles or anything like that. Magnex comes with a lifetime warranty, not sure about the k-tek warranty - Roamer will know.
Id say its down to how much noise you want really. The only other difference is the size of the pipes, which again is down to personal taste. Both are high quality. Oh, try and get a full (cat-back) system if you can afford it rather than just a back box.
Air filter - yes I got a Pipercross panel filter. It fits nicely in the airbox and will save money in the long run vs paper filters. Does it make the car faster? Who knows - only a rolling road could answer that, but I doubt its much. Overall though Im happy with my mods - I ran a 15.487 at York this afternoon which seemed respectable.
Hope this helps a bit!