We already requested DYNO diagram for the tuned 14+ intake from Injen. By the way it's non-road legal, so it seems to be a real number.
If you're looking for something more affordable - K&N or AFE filter will do the job. The difference is not that big but comparison tests showed that AFE Pro Guard 7 Air Filter
provides the best performance and filtration even after numerous cleanings.
You can check performance filters over here: Universal Air Filters
No you can either attach that filter to the aftermarket intake tube, or simply connect it to your stock tube. In both cases make sure to measure the diameter of the tube and get matching size filter.
Not sure about legality of such custom intake system...you may have problems passing vehicle inspection. On the other hand most of complete air intakes come with EQ label which shows that they're street legal.