Does more HP mean more gas mileage?
 

Can it result in more mileage? (thinking about upgrading my prelude Si to a Vtech) will I get more mileage if I upgrade my stock air intake to cold air?

No,usually it means less mileage.It takes more hay to feed the horse the bigger it gets.

what mostly happens when increasing hp is that one enjoys the extra power and uses it, thereby increasing fuel consumption, especially if the weight/power ratio initially was not satisfactory .