ULEV models have a more restrictive exhaust manifold, this manifold stay hotter and burns more fuel simaliar to a precat.

Swapping to a standard LEV manifold will return you to a full 150hp. Or just add a header and you are back on even par.

F23A1 -- standard Vtec sohc (EX/LX)
F23A4 -- ULEV Vtec
F23A5 -- non -Vtec 135hp DX sedan engine (some VP models too)