Could lose up to 10% but not necessarily. Most engines are not optimized so if you had a "normal" bracket engine at 13.5 and then you switched to a pro built pump gas combo you might not lose any power at all. All depends on the details. I've run pump gas engines on the dyno that made more power than high compression bracket engines. That actually happens on a regular basis, especially if the bracket guy picked his own cam and/or ported his own heads.