Have a look at this site - some good stuff (I think it's a forum member)...

http://www.freewebs.com/pbsuperti/index.htm

He has plugs and leads etc. - he claims 10 hp.

I agree with dynamicS... out your money elsewhere.
However, if it's time to change plugs and leads, then why not upgrade.
Or... do the suspension, then you can go round corners using all the extra power you've gained!!!