My other SVX with full exhaust was doing really good it could push 500 miles a tank on the highway. But, i removed the stock pre-cats and redid the pipe from the headers with new clean straight pipe, then put a Y-pipe hi-flow cat that went from 2 to 1 then I put in another 2.5" hi flow cat to keep the sound down, deffinately a lot nicer, and engine temps dropped quite noticably under the hood, i think it had a lot of carbon build up in those pre-cats....also, I believe i used that "Run RIght" fuel system cleaner that you attach to air compressor and your fuel rail to remove cabron from pistons and valves.....made a nice difference...

Just some ideas....