No change in compression. It takes internal engine mods to do that. Streight pipes will tend to make a bike run leaner. Pull your plugs and check the color of the center. Ideal is a tan/brown color, dry black is rich, wet black is gas/oil fouled and white is lean. Running too rich or too lean will hurt the engine in the long run. If your start up problems involve hard starting, you most likely need to rejet your carb/s due to the pipe change.