The purpose of the tumbler valve is like a choke on other vehicles, it aids in fuel air delivery during cold startups. By doing a delete, you increase the air flow through the intake manifold which increases HP.
I did a tumbler delete myself on my REX. The process wasnt hard to do. I cant tell you what the HP gain was officially, but a gain was noticed.
The APS unit is a super nice unit and with the extra injectors you can acomplish alot of different possibilites. Just remember, you have to have an aftermarket ECU to do either mod, yourself or APS.