it can add a little bit of power but nothing like a correct port job will because you will be raising all the ports and adding deck clearance you cant remove. Even with trimming the heads the cylinder will sit higher above the piston at tdc then before. The added deck clearance is not something thats desireable it ruins the way the heads works.
shimming and cutting the heads will add a few but less then 10hp for sure. You get more of the power gain on 98-99 srx 600 and 700 from adding compression, they were lower in those years.