Obviously take with a large pinch of salt as different rolling roads give different figures and no before run to see the difference but pretty happy with that figure for a 258bhp car with an exhaust.
Very strong result, but it is common knowledge these cars all tend to produce circa 275ish on rolling roads, its fair to say on a turbo car a decent exhaust will give 10-20HP also, I'd say your car is probably crying out for a tuning box maybe, ONLY DTUK may I advice, Toyota will also fit this brand as well under warranty, suspect you would be around 310-320HP then on same dyno in same conditions. May I ask where you ran the car? Also I've noticed as the miles have gone on mine it now feels stronger.
It is a shame proper tuning is so limited, but SVM I think are running around 415HP now, can only imagine how crazy that must feel in such a small light car.