1. Gears will only hurt your fuel economy if you do a lot of highway driving because of the increased cruising rpms. If you're just putting around town at 50mph or less, you really won't see a change in mpg at all. I've heard claims of 2-3 mpg loss after gears at the very most. 2. No matter what people will say, speed density can handle some amount of engine modification. With some tweaking, I've even seen it handle H/C/I. While it is a good idea to run MAF when you really start to open the engine up, a TB is not going to overcome the SD. The modifications you see in my sig work just fine on my SD car, and I have no problems with lean operation. Why are you so against it? Its not like its a step in the wrong direction, and it will open up when he does decide to go with a better intake and/or heads. If he is going to do it sooner or later, he might as well do it now, seems to me. Sounds like his heart is set on it, haha, so let him be!