Well I am not sure that just a bigger turbo will get you to the power output you are dreaming about. A quick Google puts the power output of the M400 at around 425bhp, so to get another 300bhp plus will be expensive. There are some great people on here who will know exactly what to do, but I will stick my neck out from my own experience and suggest the following. Change the fuel injectors to a higher flow version, the fuel pump will need to be increased as well to keep up with demand. A custom remap done on a rolling road will need to be completed and this should see you quite comfortably into the 600's. A bigger turbo sounds good but have you got the space for it. Remember you will need to have a new turbo manifold made that will allow the new turbo to fit. Possible that you might need to factor in an external oil pump and oil cooler to make sure the turbo cooling is taken care off. If space is limited then there is a good chance you can have the turbo removed and have it converted into a hybrid to try and get you the benefits of a bigger turbo but without the difficulties in trying to fit a huge monster into a tight gap. Plus as you said, SA is ruddy hot so power developed will not be consistent from one day to the next, so a visit to a rolling road on a warm Monday might be vastly different to a visit again on the Friday if the weather is really hot. Now all that I have suggested does not take into account a full engine strip down and rebuild with performance parts. Do you have another car to drive around in while your car is of the road for a long time getting the works done? One set of performance upgrades on my car had it off the road for a shade under 3 months.
Good luck anyway, I am sure when some of our real experts add their pearls of wisdom you will know what to do next, be warned though, deep pockets and bulging wallets are required, shattering performance does not come cheap.