If we go just by the speedometer rather than GPS or laser timing rigs, I've had decidedly north of 200km/h from a 106 BHP car (a Megane Sport, which is maybe more aerodynamic than a Corsa but has a larger frontal area/body skin area and more weight) which what sound like similar gear ratios. I think officially it was rated for somewhere in the low 190s, but it could nudge 200 on the clock on the flat and definitely continue past 210 downhill.
In any case, even if the car is only officially rated for 186km/h, not being able to clear 150 is really bad for a 100HP model. That's more like what you'd expect from something with half the power. My first car, a 45HP, 4-speed Polo with the aerodynamic finesse of a brick with a corner chipped off, officially made 145 and could be routinely coaxed to 150+ on the dial (and its higher powered brethren were stated as making, for example, about 155km/h for the 55HP model). I've had or driven 70ish HP Astra, Clio, Punto that were all happiest in the low 160s but could be eked higher and one was known to clear 190. And I've had a brace of 80ish horsepower machines, the slowest of which caps out at 160 dead (GPSed at exactly 155; fwiw, it'll do it in both 4th and 5th, without nearing redline in 4th, which makes me wonder just how horrifically undergeared that supposedly 100HP Corsa is) because for some stupid reason that's what its built in electronic limit nannies it at, even though the official spec says somewhere in the low 170s. The other two could sit at 180 on the dial all day long, varying a bit up and down with gradients.
So the question is, is there an issue with the car's power output, or is it also being limitered out by some kind of electronic cap? It sounds almost like it's only running on two cylinders out of four. But some kind of parental or fleet decision (or even the influence of an insurer's "black box") may be deliberately capping the speed. It's a fairly easy one to determine... we just need some hills. The discrepancy between claimed power and speed is wide enough that the effect should be immediately obvious.
If the car is genuinely only making about 50HP and in need of a mechanic, it'll make 150k on the flat but immediately start losing speed up even a relatively mild hill, and if you point it at some of the steepest to be found on a motorway or autobahn you'll likely have to downshift to save from slowing down too much, and it could well dip noticeably below 130k. Travelling the other way down that same hill you should be able to gather quite a bit of extra speed, maybe exceed 160.
If it's making the advertised power but is being electronically shackled, you'll find it makes almost exactly the same speed on the flat... up any hill up to a certain gradient, after which the speed will only fall off a little (a 100HP machine should still be capable of over 140k on the steepest grades any regular autobahn type road can offer)... and downhill as well, save for any descent steep enough that it will still gather speed without the accelerator being pressed. In which case you'll find that the difference between full throttle, and no throttle, is zero - the pedal has no effect on the car's speed or acceleration at all. If you floor it and push the clutch in, the revs shouldn't rise at all beyond where they would be at 150k... and indeed if you can traverse through that speed, up or down, with the clutch in and accelerator floored, you may see the rev counter move through that point at that speed, on its way to or from idle speed (at any road speed much above 150) vs banging off the limiter (any speed much below it).
In that case, what you need is an electronic exorcist to find and disable the cap ... assuming it's not a condition of your insurance or similar. And if you find some to do that, send them to me so I can unlock the last 15 or so k's out of my bloody Micra...
