Can you post more info on the shifter bushings ? May make more sense since this problem is RPM dependant. Maybe the shifter bushings from the factory flex to much at higher rpms and quick shifting causing the car to not shift into gear properly. If i recall I had a similar issue on an old prelude of mine (which used cable shifters)
All I know about them is that it seems about a stiff as a rubber band, and I would like to replace it with something about as stiff as a metal disc. However, I cannot figure out how these can be taken out and replaced with something else. Maybe someone will come out with a stiffer cable kit that we can drop in and call it a day.
OR.... someone finds out the Vauxhall has a different set of cables that may actually be better.
On another note,
I haven't ground since I topped up the transmission with half a bottle of the LUCAS OIL TRANSMISSION FIX. I believe it seems a tiny bit more stiff to shift than before, and that small amount of delay is likely helping the most. Keep in mind I have the RS and the difference in 1-2 is less than the difference for the other models.
I think my next attempt to solve the grind will be to try the Royal Purple MaxGear 75w90
After many comparisons against spec sheets of various different brands of oil, I found it seems like it should be the thickest oil and should help stop the grind. It is available at most parts stores and should be easy enough to find.
Royal Purple Max Gear Product Sheet