the 2.x ohm rating is to make the final ohm rating with the speaker and wire an actual 4ohm rating. Where typically your 4ohm speaker has 4ohms resistance in itself, you add wire- and tada.. your getting closer to 5 (not much closer, maybe 4.1-4.3). So, when they say you'll get more power from your stock amp, you will- sort of. You'll actually be getting closer to the true 4ohm rating.
So figure 2.4 ohm speaker (i think thats close to their rating, and yes that's nominal- you're prolly closer to 3.5) + 15ft of wire = 3.5-4 ohms
big differences? Most likely no, atleast to an untrained ear.