Once you get past the point your at the HP to $ ratio starts a steep decline.
Contact Coalshedracing on here. He has done some combo's that arent terribly expensive and worked very well. I reccomend a stage2 hotcams roughly 149$ and springs 150$ or so. A wiesco 11:1 is a great piston and the least expensive to boot. Anymore extreme mods your looking at replacing atleast the rod. I consider the stock rod borderline with anything other than a pipe.
None imo if you go with a piston to. If you dont want to remove the head and cylinder the stage 1 is a drop in.It is a little gain. Dont expect the same gains you saw with the exhaust but it does gain some. Eventually most people end up upgrading to the stage 2 and piston at a minimum.