In that case it probably isnt worth bothering. They are designed to drop 30mm with the weight of a 2 litre. With yours being a smaller block engine, you would be lucky if you got anywhere near that. Anything less than 30mm and you wont even notice the difference really.
There is no stopping you using them but there are always cheap springs coming up on here anyway or ebay, so keep an eye out for the ones for your car.
The point missed here is they are -30mm from a std gsi, which is already a lot lower than a astra merit anyway, ay worst they are going to be at least as low as a std gsi but I would say they will drop from even this.
I put std GSI eibach springs bilstein shocks on my astra merit, drop was around 2 1/2 to 3".