There are a lot of really good singles out there now. A few years back this was not the case and to be able to make/use 400+ HP twins were necessary. Now you can run a single that you can tow with and have 500-600 HP.
Twins are still going to keep the egt's down better, and they are going to be able to provide more topend (when using a reasonable single). But, bottom end is normally less than a good single. Also with the injectors you are currently running going to twins won't gain too much HP..... But to give you an idea, install some mach 6's and use your turbo and you will see around 500 HP. Install a good set of twins and hp will be 600-630.
Doug