A good port and polish in the heads may or may not net a few ponies, and is worth a try, thouh the heads are alrady pretty good. One thing may be to have larger valves put in the stock heads (valves from a 3L Duratech)
As far increasing displacement, there is no easy way to stroke the crank, so that is kinda out, and the stock 2.5L block won't take any overboring without new liners, which is not an easy tihng to do either. The choice way to increase displacement lately is using a 3L duratec block from a taurus or escape (or other applications) and going from there; check out the 3L Forum for some more info on the swap. Higher compression could help, but it brings with it a host of tuning issues, as the stock CR is pretty high already. My ideal setup in a contour? a 3L long block (the 3L has larger valves compared to the 2.5L), extrude hone the intake and exhaust ports, run SVT cams, (or ST220 or custom, if you can find them) and a heavily extrude honed intake manifold (or a larger custom unit that will fit under the hood) then of course because of how I am, I would want to add a turbo
As far as the stock heads on a 5L, yeah, they are far from perfect, and can greatly benefit from a good port job.
