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I'll guess the first dyno was optimistic. At least in the US, the best stock 93 octane dyno is around 350 rwhp. There is a dyno database you can explore. Maybe UK gas is a little better and allows another degree of timing or maybe not.
Your second dyno seems possible but even that seems on the high side by US results in the dyno database for just an axle back and filter. The track pipe or at least a high flow cat midpipe should do more than an axle back and filter put together. Another good mod is an underdrive pulley.
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