The main difference between the SOHC and DOHC is just that. One has 4 more valves and 1 more cam. The SOHC is only a small 15hp less than a DOHC. You can get that back by removing some accesories. I.E. switching to an electric fan, removing a/c, underdrive pully, etc. Boring, in small incraments, will only get you a very minute (my-noot) amount of power. So small you will not be able to feel it. You can always boost it. N/A KA's most likely won't get above 200hp with bolt-on's and small things alone. You need to spend the big bucks to get more. Boosting is the way to go for KA's if your looking for easy 200+ hp. Plus, you will keep the torque (which the SR doesn't have), keep your car legal (in most states), and parts are readily available for both the motors and turbo parts. Even for the KA24E.visit this website if your looking into going turbo.
http://www.jgstools.com/turbo/index2.html