I didn't think it was worth it on a petrol engine; isn't the net gain offset by the added weight and inconvenience of plumbing it in? - You've already noticed that it's mainly on diesels (where you might need to play with fuel temperature to get it to start in the first place, and I would assume if you get it too hot it pre-ignites).
The only time I've seen it on a petrol car is the Lateral Performance Imprezza, and I hate to suggest that most people aren't really in the same league as that. - That's mounted at the rear within the cut-away section behind the diff (I think).
Having said that, if you want to try it and it works, let us know about it. - It sounds like you've been doing some research, and sometimes the empirical evidence is more valuable that the theory.