Straight through will add more power although 15bhp is questionable and as for the plugs and leads they will ultimatley give a better spark which can lead to a cleaner burn but as JustGav says you will only really gain if the old one's are shot! As for induction kit's, in my opinion, as long as you use a good filter, heat sheild, cold air feed and even a stainless pipe from the filter to the inlet manifold then yes you sholud get an increase.
Ad, saw Mike last night, sorted squeeking!!! And we we're discussing your spats. Who did you get to spray them??? If you want them done better, for a small fee I will get them right!!!