you've already asked this question...... and got an 8 page response.........
i don't think the answers will change.....
no, you won't see a noticable difference. you might get a little better throttle response when you you pin it (WOT) but thats it.
i put one on the YJ (because the stock airbox doesn't fit anymore
and didn't see any change whatsoever......
save the 500$ for gas...... buy some oil futures now and hedge against the upcoming gas price increases, that'll protect you somewhat against 1.50/L