I've never noted more than a 200-300rpm change in rpm for every 5% nitro I add to the fuel. Keep in mind the combustion chamber shape has a lot to do with how an engine responds to nitromethane. AFAIK, European engines (including many ASP models) are designed for low nitro (usually 5% max) so using no nitro at all won't show such a power deficit using FAI type fuels whereas engines designed to use more nitro (OS for example) will show a higher power deficit when not using nitro. Increasing the compression ratio and changing the combustion chamber shape can regain some lost power when not using nitro but if the engine doesn't use a button head, you'd have to make a whole new head which for most folks isn't easy or cheap to do.